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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?

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.

And:

Strict mode helps out in a couple ways:

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):

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).

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:

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.

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

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

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

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

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

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

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

Forbids the with keyword

eval in strict mode does not introduce new variables

Forbids deleting plain names (delete x;)

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

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. )

arguments.callee is not supported

[Ref: Strict mode, Mozilla Developer Network]

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:

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.

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.

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:

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.

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.

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

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

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

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.

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.

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.

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.

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.

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.

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

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,

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.

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.

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):

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.)

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:

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

Sample

Eliminates with statement :

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

Sample

Prevents Duplicates :

When we have duplicate property, it throws an exception

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

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

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).

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

"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.

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

Octal syntax

this point to the global object.

Declaring twice the same name for a property name in an object literal

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

Declaring two function arguments with the same name function

Setting a value to an undeclared variable

Using delete on a variable name delete myVariable;

Using eval or arguments as variable or function argument name

Sources:

Transitioning to strict mode on MDN

Strict mode on MDN

JavaScript’s Strict Mode and Why You Should Use It on Colin J. Ihrig's blog (archived version)

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.

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.

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

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.

"use strict" makes JavaScript code to run in a strict mode, which basically means everything need to be defined before using them, the main reason for strict mode is avoiding accidental global usages of any undefined methods.

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

"use strict" is widely need to be used in ECMA5, in ECMA6 it's part of javascript by default, so don't need to be added if you 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

2) whole-script strict mode

3) Assignment to a non-writable global

For more info, visit this page here

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:

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.

"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.

Small example to compare:

Non-strict mode:

Strict mode:

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

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

The string "arguments" cannot be used as a variable:

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.

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

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.

JavaScript “strict” mode introduces in ECMAScript 5.

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.

Just wanted to add some more points.

The Reason to Use 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 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.

The things that will throw errors in Strict Mode
Using a variable, without declaring it, is not allowed:

Objects are variables too.

Using an object, without declaring it, is not allowed:

Deleting a variable (or object) is not allowed.

For security reasons, eval() is not allowed to create variables in the scope from which it was called:

In function calls like f(), the this value was the global object. In strict mode, it is now undefined.

"use strict" is only recognized at the beginning of a script.

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