What does "use strict" do in JavaScript, and what is the reasoning behind it?

6 893

1 430

Recently, I ran some of my JavaScript code through Crockford's JSLint, and it gave the following error:

Problem at line 1 character 1: Missing "use strict" statement.

Doing some searching, I realized that some people add "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. Unfortunately, Google did not reveal much of the history behind this string statement. Certainly it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.

So what is "use strict"; all about, what does it imply, and is it still relevant?

Do any of the current browsers respond to the "use strict"; string or is it for future use?

Mark Rogers

Posted 2009-08-26T16:10:13.827

Reputation: 40 900


4 530

This article about Javascript Strict Mode might interest you: John Resig - ECMAScript 5 Strict Mode, JSON, and More

To quote some interesting parts:

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.


Strict mode helps out in a couple ways:

  • It catches some common coding bloopers, throwing exceptions.
  • It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
  • It disables features that are confusing or poorly thought out.

Also note you can apply "strict mode" to the whole file... Or you can use it only for a specific function (still quoting from John Resig's article):

// Non-strict code...

  "use strict";

  // Define your library strictly...

// Non-strict code... 

Which might be helpful if you have to mix old and new code ;-)

So, I suppose it's a bit like the "use strict" you can use in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.

Currently, it's supported by all major browsers (bar IE 9 and below).


Posted 2009-08-26T16:10:13.827

Reputation: 332 751


@m4bwav : I don't think it's well supported : the page about ECMAScript 5 support in Mozilla is quite poor : https://developer.mozilla.org/En/JavaScript/ECMAScript_5_support_in_Mozilla -- and Mozilla's engine is generally not the worse one ^^

– Pascal MARTIN – 2009-08-26T16:30:46.330

3"use strict";, if IE 7+ support is not requirement, is a good practice to write better javascript. – Rui Marques – 2012-04-13T20:58:09.610

4I am using "use strict" during development and will remove during production. – kiranvj – 2012-08-07T05:54:21.593

80Changing the default after so many years ? Too late for that : it would break so many existing sites/scripts/applications... The only possible thing is to help make things better, for the future. – Pascal MARTIN – 2010-03-04T21:54:49.273

On Chrome, if the Javascript console is open "use strict" will cause Chrome to throw reference errors if using a variable that wasn't declared with var. – Pace – 2013-06-11T14:35:42.763


Everything about the "use strict"; - MDN: Strict mode

– Fabio Phms – 2013-06-20T12:05:47.983

so can i use "use strict"; at the top of the js file without the (function()... right? FYI i am new to js – Tian Loon – 2014-01-27T10:30:25.440

11@Julius - This couldn't have been implemented using a reserved keyword, because then code trying to trigger strict mode would break in old browsers. Adding a "random" string literal doesn't break anything. – nnnnnn – 2014-03-05T11:22:51.477

@PascalMARTIN "Too late for that : it would break so many existing sites/scripts/applications..." I'm not sure I see the same problem you do. – Louis – 2014-03-08T04:58:35.607

2@Louis there's an old saying "If it aint broke, don't fix it" – theycallmemorty – 2014-04-10T12:46:11.030

2@SpYk3HH People who are already Javascript experts aren't the target audience, because you're unlikely to make the kinds of stupid mistakes that use strict is supposed to prevent. It's like if Bjarne Stroustrup was talking about how he never turns on warnings in g++. Well, duh. – Chris Hayes – 2014-07-24T21:19:25.763

3@SpYk3HH If your js is professional, there should be no reason to not use "strict", and using it can only protect your pristine code from less professional meddling fingers. – ProfK – 2014-12-13T04:57:13.370

14I tried a small code snippet that would be invalid when using "use strict" in Firefox 3.6, Safari 5, Chrome 7 and Opera 10.6 (all Mac). No errors whatsoever, so i guess 'use strict' is not supported in any browser yet. Didn't test in IE9 though ;) – Husky – 2010-11-10T09:54:12.517

1@SpYk3HH I'll help you; there is nothing wrong with disagreement, after all. And I do disagree; JS is neither free nor open. EMCAscript is an open standard, but Javascript and JScript are both proprietary implementations. And if you've ever tried to get a transcript of the standard process, you'll find it isn't free either! Regardless, just because you can do something doesn't mean you should; look at how many implementations have used eval to parse json rather than an actual json parser, opening tons of sites to injection attacks. – Alice – 2015-03-02T00:39:16.517

@Alice Sorry, can't say I've ever had to use strict. I've seen applied to my libs afterward, but none of my code was changed. What can I say, when I write professionally, it's just not needed, except maybe to give someone else comfort. – SpYk3HH – 2015-03-02T01:49:38.713

5@SpYk3HH Then all that means is that you already used the correct idioms. It's fine to be incidentally correct, but it's always better to validate. That's what "use strict" does. It validates you used the correct idioms. Would you rather be incidentally correct, or provably correct? – Alice – 2015-03-02T01:56:33.967


Recent developments suggest the use of "use strong" as proposed by Google's R&D team on V8: https://developers.google.com/v8/experiments -- This would force a feature subset policy to remove certain ambiguity for performance reasons

– Dynom – 2015-03-14T16:00:03.593

@Alice I've read much of the conversation above and, also being a self-taught programmer, I completely agree with you. Provably correct > probably correct. – minexew – 2015-04-06T09:49:05.923

does 'use strict' requires ===, makes == into ===, or none of the above? – pvinis – 2015-07-02T11:00:02.907

@pvinis: None of the above. Strict mode does not affect functionality beyond throwing more errors and changing the default this. – Ry- – 2015-08-12T04:30:50.120

10Quick update: Firefox 4 has complete support for strict mode, and as far as I can tell, no other browser does. Safari and Chrome have "partial" support, but I don't really know what that means. – Sasha Chedygov – 2011-02-08T02:02:24.470

Note that if you add "use strict"; at the file level, it can cause errors when you bundle your file with 3rd-party libraries. To avoid this, use it at the function-level. – xdhmoore – 2016-03-11T22:58:38.623

5To people reading these comments: there's someone above who says they use strict mode in development but remove it for production. Doing that isn't a good idea: code that executes in strict mode may run differently in non-strict mode. Strict mode isn't a subset of non-strict mode, it changes the way your code runs. – gcampbell – 2016-06-14T13:22:43.307

@PascalMARTIN "too late", huh? The internet has been big for about 20 years. Until humanity is extinct, there will be (hopefully) another few decades. Something tells me that in 2100, there will be no more Javascript. – Blauhirn – 2016-11-05T22:29:29.953


Chrome 11 seems to pass all of these tests as does IE10 http://ie.microsoft.com/testdrive/HTML5/TryStrict/Default.html#

– gman – 2011-05-13T17:27:29.700

1@gcampbell Do you have an example of code that works differently in strict and non-strict mode, assuming that no errors/exceptions are experienced in strict mode? – Kevin – 2017-03-06T12:20:09.323


Strict mode may also considerably improve the performance http://stackoverflow.com/questions/38411552/why-use-strict-improves-performance-10x-in-this-example

– Afanasii Kurakin – 2017-04-01T07:03:58.010

1@kiranvj Please do not remove "use strict" when distributing the production version. A fundamental thing the "use strict" mode does that greatly amplifies performance across the board is it tells the browser no "eval" statements will introduce new variables into the scope. Whether or not you actually use "eval" (which you should never ever do) does not matter. What matters is failure to "use strict" prevents the browser from making that assumption because what if a property you call is eval in disguise? It probably is not, but the browser cannot make that assumption as it could break some code – Jack Giffin – 2018-05-04T20:08:24.107

@Kevin The linked performance question shows that use strict changes the value of this in certain contexts. e.g. calling (42).myNumberMethod() results in this being a Number for strict and an Object otherwise – Marie – 2018-07-06T13:41:13.480

1 138

It's a new feature of ECMAScript 5. John Resig wrote up a nice summary of it.

It's just a string you put in your JavaScript files (either at the top of your file or inside of a function) that looks like this:

"use strict";

Putting it in your code now shouldn't cause any problems with current browsers as it's just a string. It may cause problems with your code in the future if your code violates the pragma. For instance, if you currently have foo = "bar" without defining foo first, your code will start failing...which is a good thing in my opinion.


Posted 2009-08-26T16:10:13.827

Reputation: 30 697

278Fail fast and fail loudly. – Niels Bom – 2013-01-29T22:20:19.267

24If you are writing Javascript inline in HTML files, start each new block with <script>"use strict";. The flag only applies to the block in which it is included. – nobar – 2013-10-05T18:50:13.590

ECMAScript5 strict mode(in my blog). old browsers ignore it. It works like u running in without it. – Zaheer Ahmed – 2014-02-15T20:04:35.887

5It's funny, this resulted in strings must have single quotes. So write 'use strict'; instead – nilsi – 2015-01-30T08:49:17.750

1then what would happen to the hoisting concept of javascript ? – Sunil Sharma – 2015-05-14T08:33:16.217

1@SunilSharma If you try to hoist, but it fails because the variable isn't defined, at the moment it will add it to the global object. With "use strict";, it will fail instead. This makes more sense, because if it's adding it to the global object that means that it might not work the next time you run the function / do something else that resets the block, as it will be in the highest block (global). – wizzwizz4 – 2017-02-16T21:15:04.680

2When I'm adding "use strict"; to the top of my JavaScript code, I am asked to use the function form of 'use strict' instead. Can you explain why is better to use the function form of 'use strict'? Thank you – Andrew Lam – 2017-06-03T04:24:28.113


The statement "use strict"; instructs the browser to use the Strict mode, which is a reduced and safer feature set of JavaScript.

List of features (non-exhaustive)

  1. Disallows global variables. (Catches missing var declarations and typos in variable names)

  2. Silent failing assignments will throw error in strict mode (assigning NaN = 5;)

  3. Attempts to delete undeletable properties will throw (delete Object.prototype)

  4. Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"})

  5. Function parameter names must be unique (function sum (x, x) {...})

  6. Forbids octal syntax (var x = 023; some devs assume wrongly that a preceding zero does nothing to change the number.)

  7. Forbids the with keyword

  8. eval in strict mode does not introduce new variables

  9. Forbids deleting plain names (delete x;)

  10. Forbids binding or assignment of the names eval and arguments in any form

  11. Strict mode does not alias properties of the arguments object with the formal parameters. (i.e. in function sum (a,b) { return arguments[0] + b;} This works because arguments[0] is bound to a and so on. )

  12. arguments.callee is not supported

[Ref: Strict mode, Mozilla Developer Network]


Posted 2009-08-26T16:10:13.827

Reputation: 10 162

28Nit: global variables are allowed, just have to be explicit (e.g. window.foo = bar). – gcampbell – 2016-06-20T09:41:03.380

1Requires all property names in an object literal to be unique (var x = {x1: "1", x1: "2"}) is this valid – Arun Killu – 2017-07-29T08:26:18.827

1Your example in 11 is missing a modification of a (otherwise it makes no sense). I. e. function sum(a,b) { a = 0; return arguments[0] + b; } alert(sum(1, 2)) will return 3 with strict mode and 2 without strict mode, due to aliasing. – David Gausmann – 2018-03-14T11:04:03.103


If people are worried about using use strict it might be worth checking out this article:

ECMAScript 5 'Strict mode' support in browsers. What does this mean?
NovoGeek.com - Krishna's weblog

It talks about browser support, but more importantly how to deal with it safely:

function isStrictMode(){
    return !this;
   returns false, since 'this' refers to global object and 
   '!this' becomes false

function isStrictMode(){   
    "use strict";
    return !this;
   returns true, since in strict mode the keyword 'this'
   does not refer to global object, unlike traditional JS. 
   So here, 'this' is 'undefined' and '!this' becomes true.

Jamie Hutber

Posted 2009-08-26T16:10:13.827

Reputation: 13 217

111I disagree. I think this shows why its very useful. In essence it means that this returns its function and not the window – Jamie Hutber – 2013-02-26T15:25:21.207

I agree with @Erwinus... function SomeClass() { if (isStrictMode()) { } }; var foobar = new SomeClass(); – Izkata – 2013-07-10T20:02:45.930

32when do you ever want the window with this that you can't target with window? – Jamie Hutber – 2013-07-18T08:34:07.343

1So if it doesn't refer to the global variable state, what does it refer to at all? – Cole Johnson – 2013-07-28T15:51:00.023

12It refers to itself. this belongs to its own function and not the global window – Jamie Hutber – 2013-07-28T21:17:28.523

24In the second one this one is actually undefined. – Broxzier – 2013-08-14T11:40:32.787

6@JamieHutber It does not belong to its own function, you can still bind this to anything. "use strict" only disables the default behavior of assigning window to this when a this is not specified – Juan Mendes – 2013-09-07T18:34:51.823

1It's not really about finding the target, it's about getting the actual parent. Everything belongs to something in a big hierarchy in JavaScript. Obscuring that fact is not good as it will prevent people from learning the language and thus make confusing mistakes later on. – Lodewijk – 2013-09-11T17:32:38.770

1QT ''this' refers to global object' isn't correct but could be under some circumstances. – landed – 2014-02-27T13:42:08.047

Point being, without "use strict" it will always refer to the global object. – Jamie Hutber – 2014-09-26T22:17:37.863

3Jamie your last comment is incorrect. With or without strict mode this can refer to whatever object you like depending on how you call the function - that's the point of having it in the language in the first place. – nnnnnn – 2015-01-08T10:29:22.767

11The point is that your JS program will start failing due to accessing a property of an undefined, instead of silently doing the wrong thing on the global object. Makes tracking down subtle bugs much easier. – Stephen Chung – 2015-07-31T05:47:55.257

1In the second one this one is actually undefined, because there is no object on which behalf this function is being executed. Calling this window["isStrictMode"]() has exactly the opposite effect. – luke – 2016-11-04T10:57:47.557

1This answer is misleading. The value of this is set by how a function is called, so without showing the call, you can't state what the value of this is. The first function will always return false, since in non–strict mode, this must always reference an object. The second may return true or false, depending on how the function is called (and the value assigned to this by the call). – RobG – 2017-10-27T21:15:18.193


A word of caution, all you hard-charging programmers: applying "use strict" to existing code can be hazardous! This thing is not some feel-good, happy-face sticker that you can slap on the code to make it 'better'. With the "use strict" pragma, the browser will suddenly THROW exceptions in random places that it never threw before just because at that spot you are doing something that default/loose JavaScript happily allows but strict JavaScript abhors! You may have strictness violations hiding in seldom used calls in your code that will only throw an exception when they do eventually get run - say, in the production environment that your paying customers use!

If you are going to take the plunge, it is a good idea to apply "use strict" alongside comprehensive unit tests and a strictly configured JSHint build task that will give you some confidence that there is no dark corner of your module that will blow up horribly just because you've turned on Strict Mode. Or, hey, here's another option: just don't add "use strict" to any of your legacy code, it's probably safer that way, honestly. DEFINITELY DO NOT add "use strict" to any modules you do not own or maintain, like third party modules.

I think even though it is a deadly caged animal, "use strict" can be good stuff, but you have to do it right. The best time to go strict is when your project is greenfield and you are starting from scratch. Configure JSHint/JSLint with all the warnings and options cranked up as tight as your team can stomach, get a good build/test/assert system du jour rigged like Grunt+Karma+Chai, and only THEN start marking all your new modules as "use strict". Be prepared to cure lots of niggly errors and warnings. Make sure everyone understands the gravity by configuring the build to FAIL if JSHint/JSLint produces any violations.

My project was not a greenfield project when I adopted "use strict". As a result, my IDE is full of red marks because I don't have "use strict" on half my modules, and JSHint complains about that. It's a reminder to me about what refactoring I should do in the future. My goal is to be red mark free due to all of my missing "use strict" statements, but that is years away now.


Posted 2009-08-26T16:10:13.827

Reputation: 2 804

21WHY are devs in this thread so cavalier about "use strict"?? It THROWS EXCEPTIONS in otherwise working JavaScript, for goodness sakes! Just sprinkle it on the code like sugar on Corn Flakes, eh? NO! BAD! "use strict" should be used cautiously, preferably only in code you control that has unit tests that pass against all major browsers and that exercise all code paths. You got tests? Okay, "use strict" is fine for you, knock yourselves out. – DWoldrich – 2014-05-08T08:38:54.197

48Yes. Obviously "use strict" can break seemingly valid javascript which hasn't broken before. But the code not having broken before is not equal to the code being correct and doing what it's supposed to. Usually referencing undeclared variables signals a typo, etc. Use strict allows you to catch these kinds of errors, and hopefully before you ship production code. – Jostein Kjønigsen – 2015-03-09T07:46:42.067

2... or just apply "use strict" as part of a last pass over your code, fix all the obvious problems, shrug, say "good enough," then take it out for production :) – Wolfie Inu – 2015-11-10T08:49:11.333

11Personally, I never/very rarely add "use strict"; to existing code. That being said, I'll almost always use it when I'm writing new code from scratch – Martin – 2016-03-25T20:48:41.567

3If you're already using JSLint, you've probably fixed most of the places where "use strict" would break things, though. – Jonathan Cast – 2017-12-04T15:22:23.853


Using 'use strict'; does not suddenly make your code better.

The JavaScript strict mode is a feature in ECMAScript 5. You can enable the strict mode by declaring this in the top of your script/function.

'use strict';

When a JavaScript engine sees this directive, it will start to interpret the code in a special mode. In this mode, errors are thrown up when certain coding practices that could end up being potential bugs are detected (which is the reasoning behind the strict mode).

Consider this example:

var a = 365;
var b = 030;

In their obsession to line up the numeric literals, the developer has inadvertently initialized variable b with an octal literal. Non-strict mode will interpret this as a numeric literal with value 24 (in base 10). However, strict mode will throw an error.

For a non-exhaustive list of specialties in strict mode, see this answer.

Where should I use 'use strict';?

  • In my new JavaScript application: Absolutely! Strict mode can be used as a whistleblower when you are doing something stupid with your code.

  • In my existing JavaScript code: Probably not! If your existing JavaScript code has statements that are prohibited in strict-mode, the application will simply break. If you want strict mode, you should be prepared to debug and correct your existing code. This is why using 'use strict'; does not suddenly make your code better.

How do I use strict mode?

  1. Insert a 'use strict'; statement on top of your script:

    // File: myscript.js
    'use strict';
    var a = 2;

    Note that everything in the file myscript.js will be interpreted in strict mode.

  2. Or, insert a 'use strict'; statement on top of your function body:

    function doSomething() {
        'use strict';

    Everything in the lexical scope of function doSomething will be interpreted in strict mode. The word lexical scope is important here. See this answer for a better explanation.

What things are prohibited in strict mode?

I found a nice article describing several things that are prohibited in strict mode (note that this is not an exclusive list):


Historically, JavaScript has been confused about how functions are scoped. Sometimes they seem to be statically scoped, but some features make them behave like they are dynamically scoped. This is confusing, making programs difficult to read and understand. Misunderstanding causes bugs. It also is a problem for performance. Static scoping would permit variable binding to happen at compile time, but the requirement for dynamic scope means the binding must be deferred to runtime, which comes with a significant performance penalty.

Strict mode requires that all variable binding be done statically. That means that the features that previously required dynamic binding must be eliminated or modified. Specifically, the with statement is eliminated, and the eval function’s ability to tamper with the environment of its caller is severely restricted.

One of the benefits of strict code is that tools like YUI Compressor can do a better job when processing it.

Implied Global Variables

JavaScript has implied global variables. If you do not explicitly declare a variable, a global variable is implicitly declared for you. This makes programming easier for beginners because they can neglect some of their basic housekeeping chores. But it makes the management of larger programs much more difficult and it significantly degrades reliability. So in strict mode, implied global variables are no longer created. You should explicitly declare all of your variables.

Global Leakage

There are a number of situations that could cause this to be bound to the global object. For example, if you forget to provide the new prefix when calling a constructor function, the constructor's this will be bound unexpectedly to the global object, so instead of initializing a new object, it will instead be silently tampering with global variables. In these situations, strict mode will instead bind this to undefined, which will cause the constructor to throw an exception instead, allowing the error to be detected much sooner.

Noisy Failure

JavaScript has always had read-only properties, but you could not create them yourself until ES5’s Object.createProperty function exposed that capability. If you attempted to assign a value to a read-only property, it would fail silently. The assignment would not change the property’s value, but your program would proceed as though it had. This is an integrity hazard that can cause programs to go into an inconsistent state. In strict mode, attempting to change a read-only property will throw an exception.


The octal (or base 8) representation of numbers was extremely useful when doing machine-level programming on machines whose word sizes were a multiple of 3. You needed octal when working with the CDC 6600 mainframe, which had a word size of 60 bits. If you could read octal, you could look at a word as 20 digits. Two digits represented the op code, and one digit identified one of 8 registers. During the slow transition from machine codes to high level languages, it was thought to be useful to provide octal forms in programming languages.

In C, an extremely unfortunate representation of octalness was selected: Leading zero. So in C, 0100 means 64, not 100, and 08 is an error, not 8. Even more unfortunately, this anachronism has been copied into nearly all modern languages, including JavaScript, where it is only used to create errors. It has no other purpose. So in strict mode, octal forms are no longer allowed.

Et cetera

The arguments pseudo array becomes a little bit more array-like in ES5. In strict mode, it loses its callee and caller properties. This makes it possible to pass your arguments to untrusted code without giving up a lot of confidential context. Also, the arguments property of functions is eliminated.

In strict mode, duplicate keys in a function literal will produce a syntax error. A function can’t have two parameters with the same name. A function can’t have a variable with the same name as one of its parameters. A function can’t delete its own variables. An attempt to delete a non-configurable property now throws an exception. Primitive values are not implicitly wrapped.

Reserved words for future JavaScript versions

ECMAScript 5 adds a list of reserved words. If you use them as variables or arguments, strict mode will throw an error. The reserved words are:

implements, interface, let, package, private, protected, public, static, and yield

Further Reading


Posted 2009-08-26T16:10:13.827

Reputation: 12 663


I strongly recommend every developer to start using strict mode now. There are enough browsers supporting it that strict mode will legitimately help save us from errors we didn’t even know were in your code.

Apparently, at the initial stage there will be errors we have never encountered before. To get the full benefit, we need to do proper testing after switching to strict mode to make sure we have caught everything. Definitely we don’t just throw use strict in our code and assume there are no errors. So the churn is that it’s time to start using this incredibly useful language feature to write better code.

For example,

var person = {
    name : 'xyz',
    position : 'abc',
    fullname : function () {  "use strict"; return this.name; }

JSLint is a debugger written by Douglas Crockford. Simply paste in your script, and it’ll quickly scan for any noticeable issues and errors in your code.


Posted 2009-08-26T16:10:13.827

Reputation: 6 485


@JamieHutber : Please visit this link http://caniuse.com/use-strict AND http://kangax.github.io/es5-compat-table/. It will give exact idea for all browser.

– Pank – 2014-01-18T13:21:12.953


I would like to offer a somewhat more founded answer complementing the other answers. I was hoping to edit the most popular answer, but failed. I tried to make it as comprehensive and complete as I could.

You can refer to the MDN documentation for more information.

"use strict" a directive introduced in ECMAScript 5.

Directives are similar to statements, yet different.

  • use strict does not contain key words: The directive is a simple expression statement, which consists of a special string literal (in single or double quotes). JavaScript engines, that do not implement ECMAScript 5, merely see an expression statement without side effects. It is expected that future versions of ECMAScript standards introduce use as a real key word; the quotes would thereby become obsolete.
  • use strict can be used only at the beginning of a script or of a function, i.e. it must precede every other (real) statement. It does not have to be the first instruction in a script of function: it can be preceded by other statement expressions that consist of string literals ( and JavaScript implementations can treat them as implementation specific directives). String literals statements, which follow a first real statement (in a script or function) are simple expression statements. Interpreters must not interpret them as directives and they have no effect.

The use strict directive indicates that the following code (in a script or a function) is strict code. The code in the highest level of a script (code that is not in a function) is considered strict code when the script contains a use strict directive. The content of a function is considered strict code when the function itself is defined in a strict code or when the function contains a use strict directive. Code that is passed to an eval() method is considered strict code when eval() was called from a strict code or contains the use strict directive itself.

The strict mode of ECMAScript 5 is a restricted subset of the JavaScript language, which eliminates relevant deficits of the language and features more stringent error checking and higher security. The following lists the differences between strict mode and normal mode (of which the first three are particularly important):

  • You cannot use the with-statement in strict mode.
  • In strict mode all variables have to be declared: if you assign a value to an identifier that has not been declared as variable, function, function parameter, catch-clause parameter or property of the global Object, then you will get a ReferenceError. In normal mode the identifier is implicitly declared as a global variable (as a property of the global Object)
  • In strict mode the keyword this has the value undefined in functions that were invoked as functions (not as methods). (In normal mode this always points to the global Object). This difference can be used to test if an implementation supports the strict mode:
var hasStrictMode = (function() { "use strict"; return this===undefined }());
  • Also when a function is invoked with call() or apply in strict mode, then this is exactly the value of the first argument of the call()or apply() invocation. (In normal mode null and undefined are replaced by the global Object and values, which are not objects, are cast into objects.)

  • In strict mode you will get a TypeError, when you try to assign to readonly properties or to define new properties for a non extensible object. (In normal mode both simply fail without error message.)

  • In strict mode, when passing code to eval(), you cannot declare or define variables or functions in the scope of the caller (as you can do it in normal mode). Instead, a new scope is created for eval() and the variables and functions are within that scope. That scope is destroyed after eval() finishes execution.
  • In strict mode the arguments-object of a function contains a static copy of the values, which are passed to that function. In normal mode the arguments-object has a somewhat "magical" behaviour: The elements of the array and the named function parameters reference both the same value.
  • In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier (a variable, function or function parameter). In normal mode the delete expression would do nothing and is evaluated to false.
  • In strict mode you will get a TypeError when you try to delete a non configurable property. (In normal mode the attempt simply fails and the delete expression is evaluated to false).
  • In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal. (In normal mode there is no error.)
  • In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name. (In normal mode there is no error.)
  • In strict mode octal literals are not allowed (these are literals that start with 0x. (In normal mode some implementations do allow octal literals.)
  • In strict mode the identifiers eval and arguments are treated like keywords. You cannot change their value, cannot assign a value to them, and you cannot use them as names for variables, functions, function parameters or identifiers of a catch block.
  • In strict mode are more restrictions on the possibilities to examine the call stack. arguments.caller and arguments.callee cause a TypeError in a function in strict mode. Furthermore, some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them.


Posted 2009-08-26T16:10:13.827

Reputation: 7 447

2"In strict mode octal literals are not allowed (these are literals that start with 0x ...)" octal literals start with a leading 0. – Alex Gittemeier – 2016-08-11T19:52:32.433


My two cents:

One of the goals of strict mode is to allow for faster debugging of issues. It helps the developers by throwing exception when certain wrong things occur that can cause silent & strange behaviour of your webpage. The moment we use use strict, the code will throw out errors which helps developer to fix it in advance.

Few important things which I have learned after using use strict :

Prevents Global Variable Declaration:

var tree1Data = { name: 'Banana Tree',age: 100,leafCount: 100000};

function Tree(typeOfTree) {
    var age;
    var leafCount;

    age = typeOfTree.age;
    leafCount = typeOfTree.leafCount;
    nameoftree = typeOfTree.name;

var tree1 = new Tree(tree1Data);

Now,this code creates nameoftree in global scope which could be accessed using window.nameoftree. When we implement use strict the code would throw error.

Uncaught ReferenceError: nameoftree is not defined


Eliminates with statement :

with statements can't be minified using tools like uglify-js. They're also deprecated and removed from future JavaScript versions.


Prevents Duplicates :

When we have duplicate property, it throws an exception

Uncaught SyntaxError: Duplicate data property in object literal not allowed in strict mode

"use strict";
var tree1Data = {
    name: 'Banana Tree',
    age: 100,
    leafCount: 100000,
    name:'Banana Tree'

There are few more but I need to gain more knowledge on that.


Posted 2009-08-26T16:10:13.827

Reputation: 4 238


If you use a browser released in the last year or so then it most likely supports JavaScript Strict mode. Only older browsers around before ECMAScript 5 became the current standard don't support it.

The quotes around the command make sure that the code will still work in older browsers as well (although the things that generate a syntax error in strict mode will generally just cause the script to malfunction in some hard to detect way in those older browsers).


Posted 2009-08-26T16:10:13.827

Reputation: 655

10Then what does it do? – Anish Gupta – 2012-05-20T14:47:37.340

5... this describes in part the compatibility, but not what it actually does. – courtsimas – 2012-07-11T16:04:02.177


Strict mode makes several changes to normal JavaScript semantics:

  • eliminates some JavaScript silent errors by changing them to throw errors.

  • fixes mistakes that make it difficult for JavaScript engines to perform optimizations.

  • prohibits some syntax likely to be defined in future versions of ECMAScript.

for more information vistit Strict Mode- Javascript

Renganathan M G

Posted 2009-08-26T16:10:13.827

Reputation: 2 289


When adding "use strict";, the following cases will throw a SyntaxError before the script is executing:

  • Paving the way for future ECMAScript versions, using one of the newly reserved keywords (in prevision for ECMAScript 6): implements, interface, let, package, private, protected, public, static, and yield.

  • Declaring function in blocks

    if(a<b){ function f(){} }
  • Octal syntax

    var n = 023;
  • this point to the global object.

     function f() {
          "use strict";
          this.a = 1;
  • Declaring twice the same name for a property name in an object literal

     {a: 1, b: 3, a: 7} 

    This is no longer the case in ECMAScript 6 (bug 1041128).

  • Declaring two function arguments with the same name function

    f(a, b, b){}
  • Setting a value to an undeclared variable

    function f(x){
       "use strict";
       var a = 12;
       b = a + x*35; // error!
  • Using delete on a variable name delete myVariable;

  • Using eval or arguments as variable or function argument name

    "use strict";
    var obj = { set p(arguments) { } };
    try { } catch (arguments) { }
    function arguments() { } 



Posted 2009-08-26T16:10:13.827

Reputation: 22 663


"Use Strict"; is an insurance that programmer will not use the loose or the bad properties of JavaScript. It is a guide, just like a ruler will help you make straight lines. "Use Strict" will help you do "Straight coding".

Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code.

Believe me. The overhead is negligible compared to poorly designed code. Doug Crockford, who has been a senior JavaScript developer for several years, has a very interesting post here. Personally, I like to return to his site all the time to make sure I don't forget my good practice.

Modern JavaScript practice should always evoke the "Use Strict"; pragma. The only reason that the ECMA Group has made the "Strict" mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices.


Posted 2009-08-26T16:10:13.827

Reputation: 779

65The reason strict mode is optional has nothing to do with what you've stated. The real reason is to not break existing code that may not conform. – George Jempty – 2013-10-31T13:34:21.127

17Indeed, the less experienced coders ought to be the first ones to enable the "use strict"; – Antti Haapala – 2014-08-19T05:37:20.980


Including use strict in the beginning of your all sensitive JavaScript files from this point is a small way to be a better JavaScript programmer and avoid random variables becoming global and things change silently.


Posted 2009-08-26T16:10:13.827

Reputation: 2 205


Quoting from w3schools:

The "use strict" Directive

The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript version 5).

It is not a statement, but a literal expression, ignored by earlier versions of JavaScript.

The purpose of "use strict" is to indicate that the code should be executed in "strict mode".

With strict mode, you can not, for example, use undeclared variables.

Why Strict Mode?

Strict mode makes it easier to write "secure" JavaScript.

Strict mode changes previously accepted "bad syntax" into real errors.

As an example, in normal JavaScript, mistyping a variable name creates a new global variable. In strict mode, this will throw an error, making it impossible to accidentally create a global variable.

In normal JavaScript, a developer will not receive any error feedback assigning values to non-writable properties.

In strict mode, any assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object, will throw an error.

Please refer to http://www.w3schools.com/js/js_strict.asp to know more

Mehul Singh

Posted 2009-08-26T16:10:13.827

Reputation: 490


"use strict" makes JavaScript code to run in strict mode, which basically means everything needs to be defined before use. The main reason for using strict mode is to avoid accidental global uses of undefined methods.

Also in strict mode, things run faster, some warnings or silent warnings throw fatal errors, it's better to always use it to make a neater code.

"use strict" is widely needed to be used in ECMA5, in ECMA6 it's part of JavaScript by default, so it doesn't need to be added if you're using ES6.

Look at these statements and examples from MDN:

The "use strict" Directive
The "use strict" directive is new in JavaScript 1.8.5 (ECMAScript version 5). It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. The purpose of "use strict" is to indicate that the code should be executed in "strict mode". With strict mode, you can not, for example, use undeclared variables.

Examples of using "use strict":
Strict mode for functions: Likewise, to invoke strict mode for a function, put the exact statement "use strict"; (or 'use strict';) in the function's body before any other statements.

1) strict mode in functions

 function strict() {
     // Function-level strict mode syntax
     'use strict';
     function nested() { return 'And so am I!'; }
     return "Hi!  I'm a strict mode function!  " + nested();
 function notStrict() { return "I'm not strict."; }

 console.log(strict(), notStrict());

2) whole-script strict mode

'use strict';
var v = "Hi! I'm a strict mode script!";

3) Assignment to a non-writable global

'use strict';

// Assignment to a non-writable global
var undefined = 5; // throws a TypeError
var Infinity = 5; // throws a TypeError

// Assignment to a non-writable property
var obj1 = {};
Object.defineProperty(obj1, 'x', { value: 42, writable: false });
obj1.x = 9; // throws a TypeError

// Assignment to a getter-only property
var obj2 = { get x() { return 17; } };
obj2.x = 5; // throws a TypeError

// Assignment to a new property on a non-extensible object.
var fixed = {};
fixed.newProp = 'ohai'; // throws a TypeError

You can read more on MDN.


Posted 2009-08-26T16:10:13.827

Reputation: 43 514


There's a good talk by some people who were on the ECMAScript committee: Changes to JavaScript, Part 1: ECMAScript 5" about how incremental use of the "use strict" switch allows JavaScript implementers to clean up a lot of the dangerous features of JavaScript without suddenly breaking every website in the world.

Of course it also talks about just what a lot of those misfeatures are (were) and how ECMAScript 5 fixes them.


Posted 2009-08-26T16:10:13.827

Reputation: 649


Note that use strict was introduced in EcmaScript 5 and was kept since then.

Below are the conditions to trigger strict mode in ES6 and ES7:

  • Global code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive (see 14.1.1).
  • Module code is always strict mode code.
  • All parts of a ClassDeclaration or a ClassExpression are strict mode code.
  • Eval code is strict mode code if it begins with a Directive Prologue that contains a Use Strict Directive or if the call to eval is a direct eval (see that is contained in strict mode code.
  • Function code is strict mode code if the associated FunctionDeclaration, FunctionExpression, GeneratorDeclaration, GeneratorExpression, MethodDefinition, or ArrowFunction is contained in strict mode code or if the code that produces the value of the function’s [[ECMAScriptCode]] internal slot begins with a Directive Prologue that contains a Use Strict Directive.
  • Function code that is supplied as the arguments to the built-in Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive.


Posted 2009-08-26T16:10:13.827

Reputation: 3 672


Small examples to compare:

Non-strict mode:

for (i of [1,2,3]) console.log(i)
// output:
// 1
// 2
// 3

Strict mode:

'use strict';
for (i of [1,2,3]) console.log(i)

// output:
// Uncaught ReferenceError: i is not defined

Non-strict mode:

String.prototype.test = function () {
  console.log(typeof this === 'string');


// output
// false

String.prototype.test = function () {
  'use strict';
  console.log(typeof this === 'string');


// output
// true

Tân Nguyễn

Posted 2009-08-26T16:10:13.827

Reputation: 1


The main reasons why developers should use "use strict" are:

  1. Prevents accidental declaration of global variables.Using "use strict()" will make sure that variables are declared with var before use. Eg:

    function useStrictDemo(){
     'use strict';
     //works fine
     var a = 'No Problem';
     //does not work fine and throws error
     k = "problem"
     //even this will throw error
     someObject = {'problem': 'lot of problem'};
  2. N.B: The "use strict" directive is only recognized at the beginning of a script or a function.
  3. The string "arguments" cannot be used as a variable:

    "use strict";
    var arguments = 3.14;    // This will cause an error
  4. Will restrict uses of keywords as variables. Trying to use them will throw errors.

In short will make your code less error prone and in turn will make you write good code.

To read more about it you can refer here.

Pritam Banerjee

Posted 2009-08-26T16:10:13.827

Reputation: 10 510


"use strict"; is the ECMA effort to make JavaScript a little bit more robust. It brings in JS an attempt to make it at least a little "strict" (other languages implement strict rules since the 90s). It actually "forces" JavaScript developers to follow some sort of coding best practices. Still, JavaScript is very fragile. There is no such thing as typed variables, typed methods, etc. I strongly recommend JavaScript developers to learn a more robust language such as Java or ActionScript3, and implement the same best practices in your JavaScript code, it will work better and be easier to debug.


Posted 2009-08-26T16:10:13.827

Reputation: 457


use strict is a way to make your code safer, cause you can't use dangerous features which can work not as you expect.And as was writed before it makes code more strict.

Просто программист

Posted 2009-08-26T16:10:13.827

Reputation: 132


Use Strict is used to show common and repeated errors so that it is handled differently , and changes the way java script runs , such changes are :

  • Prevents accidental globals

  • No duplicates

  • Eliminates with

  • Eliminates this coercion

  • Safer eval()

  • Errors for immutables

you can also read this article for the details


Posted 2009-08-26T16:10:13.827

Reputation: 587


Normally java script does not follow strict rules hence increasing chances of errors. After using "use strict", the java script code should follow strict set of rules as like in other programming languages such as use of terminators, declaration before initialization etc.

If "use strict" is used then the code should be written by following a strict set of rules hence decreasing the chances of errors and ambiguities.

Bikash Chapagain

Posted 2009-08-26T16:10:13.827

Reputation: 99


JavaScript “strict” mode introduces in ECMAScript 5.

  (function() {
  "use strict";
   your code...

writing "use strict"; at the very top of your JS file turns on strict syntax checking. It does the following tasks for us :

(i) shows an error if you try to assign to an undeclared variable

(ii) stops you from overwriting key JS system libraries

(ii) forbids some unsafe or error-prone language features

"use strict" also works inside of individual functions. It is always a better practice to include "use strict in your code.

Browser Compatibility Issue: The "use" directives are meant to be backwards-compatible. Browsers that donot support them will just see a String literal that isn't referenced further. So, they will pass over it and move on.

Rabin Pantha

Posted 2009-08-26T16:10:13.827

Reputation: 663