JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "sloppy mode". Strict mode isn't just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

Strict mode makes several changes to normal JavaScript semantics:

1. Eliminates some JavaScript silent errors by changing them to throw errors.
2. Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.
3. Prohibits some syntax likely to be defined in future versions of ECMAScript.

See transitioning to strict mode, if you want to change your code to work in the restricted variant of JavaScript.

Strict mode applies to entire scripts or to individual functions. It doesn't apply to block statements enclosed in {} braces; attempting to apply it to such contexts does nothing. eval code, Function code, event handler attributes, strings passed to setTimeout(), and related functions are entire scripts, and invoking strict mode in them works as expected.

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.

// Whole-script strict mode syntax
'use strict';
const v = "Hi! I'm a strict mode script!";

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.

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

In strict mode, starting with ES2015, functions inside blocks are scoped to that block. Prior to ES2015, block-level functions were forbidden in strict mode.

ECMAScript 2015 introduced JavaScript modules and therefore a 3rd way to enter strict mode. The entire contents of JavaScript modules are automatically in strict mode, with no statement needed to initiate it.

function myStrictFunction() {
  // because this is a module, I'm strict by default
}
export default myStrictFunction;

All parts of ECMAScript classes are strict mode code, including both class declarations and class expressions — and so also including all parts of class bodies.

Strict mode changes both syntax and runtime behavior. Changes generally fall into these categories: changes converting mistakes into errors (as syntax errors or at runtime), changes simplifying how the particular variable for a given use of a name is computed, changes simplifying eval and arguments, changes making it easier to write "secure" JavaScript, and changes anticipating future ECMAScript evolution.

Strict mode changes some previously-accepted mistakes into errors. JavaScript was designed to be easy for novice developers, and sometimes it gives operations which should be errors non-error semantics. Sometimes this fixes the immediate problem, but sometimes this creates worse problems in the future. Strict mode treats these mistakes as errors so that they're discovered and promptly fixed.

First, strict mode makes it impossible to accidentally create global variables. In normal JavaScript mistyping a variable in an assignment creates a new property on the global object and continues to "work" (although future failure is possible: likely, in modern JavaScript). Assignments, which would accidentally create global variables, instead throw an error in strict mode:

'use strict';
let mistypeVariable;

mistypeVarible = 17;  // Assuming no global variable mistypeVarible exists
                      // this line throws a ReferenceError due to the
                      // misspelling of "mistypeVariable" (lack of an "a")

Second, strict mode makes assignments which would otherwise silently fail to throw an exception. For example, NaN is a non-writable global variable. In normal code assigning to NaN does nothing; the developer receives no failure feedback. In strict mode assigning to NaN throws an exception. Any assignment that silently fails in normal code (assignment to a non-writable global or property, assignment to a getter-only property, assignment to a new property on a non-extensible object) will throw in strict mode:

'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
const obj1 = {};
Object.defineProperty(obj1, 'x', { value: 42, writable: false });
obj1.x = 9; // throws a TypeError

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

// Assignment to a new property on a non-extensible object
const fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = 'ohai'; // throws a TypeError

Third, attempts to delete undeletable properties throw in strict mode (where before the attempt would have no effect):

'use strict';
delete Object.prototype; // throws a TypeError

Fourth, strict mode requires that function parameter names be unique. In normal code the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through arguments[i], so they're not completely inaccessible. Still, this hiding makes little sense and is probably undesirable (it might hide a typo, for example), so in strict mode duplicate argument names are a syntax error:

function sum(a, a, c) { // !!! syntax error
  'use strict';
  return a + a + c; // wrong if this code ran
}

Fifth, strict mode forbids a 0-prefixed octal literal or octal escape sequence. Outside strict mode, a number beginning with a 0, such as 0644, is interpreted as an octal number (0644 === 420), if all digits are smaller than 8. Octal escape sequences, such as "\45", which is equal to "%", can be used to represent characters by extended-ASCII character code numbers in octal. In strict mode, this is a syntax error. The standardized way to denote octal literals is via the 0o prefix. For example:

const a = 0o10;

Novice developers sometimes believe a leading zero prefix has no semantic meaning, so they might use it as an alignment device — but this changes the number's meaning! A leading zero syntax for the octal is rarely useful and can be mistakenly used, so strict mode makes it a syntax error:

'use strict';
const sum = 015 + // !!! syntax error
            197 +
            142;

const sumWithOctal = 0o10 + 8;
console.log(sumWithOctal); // 16

Sixth, strict mode forbids setting properties on primitive values. Without strict mode, setting properties is ignored (no-op), with strict mode, however, a TypeError is thrown.

'use strict';

false.true = '';         // TypeError
(14).sailing = 'home';   // TypeError
'with'.you = 'far away'; // TypeError

Duplicate property names used to be considered a SyntaxError in strict mode. With the introduction of computed property names, making duplication possible at runtime, this restriction was removed in ES2015.

'use strict';
const o = { p: 1, p: 2 }; // syntax error prior to ECMAScript 2015

Strict mode simplifies how variable names map to particular variable definitions in the code. Many compiler optimizations rely on the ability to say that variable X is stored in that location: this is critical to fully optimizing JavaScript code. JavaScript sometimes makes this basic mapping of name to variable definition in the code impossible to perform until runtime. Strict mode removes most cases where this happens, so the compiler can better optimize strict mode code.

First, strict mode prohibits with. The problem with with is that any name inside the block might map either to a property of the object passed to it, or to a variable in surrounding (or even global) scope, at runtime; it's impossible to know which beforehand. Strict mode makes with a syntax error, so there's no chance for a name in a with to refer to an unknown location at runtime:

'use strict';
const x = 17;
with (obj) { // !!! syntax error
  // If this weren't strict mode, would this be const x, or
  // would it instead be obj.x?  It's impossible in general
  // to say without running the code, so the name can't be
  // optimized.
  x;
}

The simple alternative of assigning the object to a short name variable, then accessing the corresponding property on that variable, stands ready to replace with.

Second, eval of strict mode code does not introduce new variables into the surrounding scope. In normal code eval("var x;") introduces a variable x into the surrounding function or the global scope. This means that, in general, in a function containing a call to eval every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because that eval might have introduced a new variable that would hide the outer variable). In strict mode eval creates variables only for the code being evaluated, so eval can't affect whether a name refers to an outer variable or some local variable:

var x = 17;
var evalX = eval("'use strict'; var x = 42; x;");
console.assert(x === 17);
console.assert(evalX === 42);

If the function eval is invoked by an expression of the form eval(...) in strict mode code, the code will be evaluated as strict mode code. The code may explicitly invoke strict mode, but it's unnecessary to do so.

function strict1(str) {
  'use strict';
  return eval(str); // str will be treated as strict mode code
}
function strict2(f, str) {
  'use strict';
  return f(str); // not eval(...): str is strict if and only
                 // if it invokes strict mode
}
function nonstrict(str) {
  return eval(str); // str is strict if and only
                    // if it invokes strict mode
}

strict1("'Strict mode code!'");
strict1("'use strict'; 'Strict mode code!'");
strict2(eval, "'Non-strict code.'");
strict2(eval, "'use strict'; 'Strict mode code!'");
nonstrict("'Non-strict code.'");
nonstrict("'use strict'; 'Strict mode code!'");

Thus names in strict mode eval code behave identically to names in strict mode code not being evaluated as the result of eval.

Third, strict mode forbids deleting plain names. delete name in strict mode is a syntax error:

'use strict';

var x;
delete x; // !!! syntax error

eval('var y; delete y;'); // !!! syntax error

Strict mode makes arguments and eval less bizarrely magical. Both involve a considerable amount of magical behavior in normal code: eval to add or remove bindings and to change binding values, and arguments by its indexed properties aliasing named arguments. Strict mode makes great strides toward treating eval and arguments as keywords, although full fixes will not come until a future edition of ECMAScript.

First, the names eval and arguments can't be bound or assigned in language syntax. All these attempts to do so are syntax errors:

'use strict';
eval = 17;
arguments++;
++eval;
const obj = { set p(arguments) { } };
let eval;
try { } catch (arguments) { }
function x(eval) { }
function arguments() { }
const y = function eval() { };
const f = new Function('arguments', "'use strict'; return 17;");

Second, strict mode code doesn't alias properties of arguments objects created within it. In normal code within a function whose first argument is arg, setting arg also sets arguments[0], and vice versa (unless no arguments were provided or arguments[0] is deleted). arguments objects for strict mode functions store the original arguments when the function was invoked. arguments[i] does not track the value of the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i].

function f(a) {
  'use strict';
  a = 42;
  return [a, arguments[0]];
}
const pair = f(17);
console.assert(pair[0] === 42);
console.assert(pair[1] === 17);

Third, arguments.callee is no longer supported. In normal code arguments.callee refers to the enclosing function. This use case is weak: name the enclosing function! Moreover, arguments.callee substantially hinders optimizations like inlining functions, because it must be made possible to provide a reference to the un-inlined function if arguments.callee is accessed. arguments.callee for strict mode functions is a non-deletable property which throws an error when set or retrieved:

'use strict';
const f = function () {
  return arguments.callee;
};
f(); // throws a TypeError

Strict mode makes it easier to write "secure" JavaScript. Some websites now provide ways for users to write JavaScript which will be run by the website on behalf of other users. JavaScript in browsers can access the user's private information, so such JavaScript must be partially transformed before it is run, to censor access to forbidden functionality. JavaScript's flexibility makes it effectively impossible to do this without many runtime checks. Certain language functions are so pervasive that performing runtime checks has a considerable performance cost. A few strict mode tweaks, plus requiring that user-submitted JavaScript be strict mode code and that it be invoked in a certain manner, substantially reduce the need for those runtime checks.

First, the value passed as this to a function in strict mode is not forced into being an object (a.k.a. "boxed"). For a normal function, this is always an object: either the provided object if called with an object-valued this; the value, boxed, if called with a Boolean, string, or number this; or the global object if called with an undefined or null this. (Use call, apply, or bind to specify a particular this.) Not only is automatic boxing a performance cost, but exposing the global object in browsers is a security hazard because the global object provides access to functionality that "secure" JavaScript environments must restrict. Thus for a strict mode function, the specified this is not boxed into an object, and if unspecified, this will be undefined:

'use strict';
function fun() { return this; }
console.assert(fun() === undefined);
console.assert(fun.call(2) === 2);
console.assert(fun.apply(null) === null);
console.assert(fun.call(undefined) === undefined);
console.assert(fun.bind(true)());

Second, in strict mode it's no longer possible to "walk" the JavaScript stack via commonly-implemented extensions to ECMAScript. In normal code with these extensions, when a function fun is in the middle of being called, fun.caller is the function that most recently called fun, and fun.arguments is the arguments for that invocation of fun. Both extensions are problematic for "secure" JavaScript because they allow "secured" code to access "privileged" functions and their (potentially unsecured) arguments. If fun is in strict mode, both fun.caller and fun.arguments are non-deletable properties which throw when set or retrieved:

function restricted() {
  'use strict';
  restricted.caller;    // throws a TypeError
  restricted.arguments; // throws a TypeError
}
function privilegedInvoker() {
  return restricted();
}
privilegedInvoker();

The major browsers have fully implemented strict mode since approximately 2012, including IE since version 10, Firefox since version 4. Chrome since version 13, etc. If you still support very old JS environments prior to the roll-outs of strict mode support, be careful to test any of your code that declares strict mode code to verify that its expected behaviors aren't violated when running in non-strict mode conforming JS engines.

There are however some nuances to consider with how strict mode behaves in browsers.

Strict mode prohibits function statements that are not at the top level of a script or function. In normal mode in browsers, function statements are permitted "everywhere". This is not part of ES5 (or even ES3)! It's an extension with incompatible semantics in different browsers. Note that function statements outside top level are permitted in ES2015.

For example, these block-level function declarations should be disallowed in strict mode by the specification's text proper:

'use strict';
if (true) {
  function f() { } // !!! syntax error
  f();
}

for (let i = 0; i < 5; i++) {
  function f2() { } // !!! syntax error
  f2();
}

function baz() { // kosher
  function eit() { } // also kosher
}

However, Appendix B of the specification recognizes on-the-ground reality of how code has behaved historically in the majority of JS engines/environments, particularly JS engines used by web browsers (including the engine used by Node.js). As such, while strict mode in the specification in proper restricts function declarations not at the top level of a script or function, Appendix B's "Block-Level Function Declarations Web Legacy Compatibility Semantics" modifies (reduces or removes) this restriction for the applicable JS environments.

• Strict Mode Code in the ECMAScript specification
• Strict mode compatibility table