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 either function bodies or 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.";
}

The "use strict" directive can only be applied to the body of functions with simple parameters. Using "use strict" in functions with rest, default, or destructured parameters is a syntax error.

function sum(a = 1, b = 2) {
  // SyntaxError: "use strict" not allowed in function with default parameter
  "use strict";
  return a + b;
}

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 a class's body are strict mode code, including both class declarations and class expressions.

class C1 {
  // All code here is evaluated in strict mode
  test() {
    delete Object.prototype;
  }
}
new C1().test(); // TypeError, because test() is in strict mode

const C2 = class {
  // All code here is evaluated in strict mode
};

// Code here may not be in strict mode
delete Object.prototype; // Will not throw error

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 variable references are resolved
• changes simplifying eval and arguments
• changes making it easier to write "secure" JavaScript
• 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.

Assigning to undeclared variables

Strict mode makes it impossible to accidentally create global variables. In sloppy mode, mistyping a variable in an assignment creates a new property on the global object and continues to "work". Assignments which would accidentally create global variables throw an error in strict mode:

"use strict";
let mistypeVariable;

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

Failing to assign to object properties

Strict mode makes assignments which would otherwise silently fail to throw an exception. There are three ways to fail a property assignment:

• assignment to a non-writable data property
• assignment to a getter-only accessor property
• assignment to a new property on a non-extensible object

For example, NaN is a non-writable global variable. In sloppy mode, assigning to NaN does nothing; the developer receives no failure feedback. In strict mode, assigning to NaN throws an exception.

"use strict";

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

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

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

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

Failing to delete object properties

Attempts to delete a non-configurable or otherwise undeletable (e.g. it's intercepted by a proxy's deleteProperty handler which returns false) property throw in strict mode (where before the attempt would have no effect):

"use strict";
delete Object.prototype; // TypeError
delete [].length; // TypeError

Strict mode also forbids deleting plain names. delete name in strict mode is a syntax error:

"use strict";

var x;
delete x; // syntax error

If the name is a configurable global property, prefix it with globalThis to delete it.

"use strict";

delete globalThis.x;

Duplicate parameter names

Strict mode requires that function parameter names be unique. In sloppy mode, the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through arguments, 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
}

Legacy octal literals

Strict mode forbids a 0-prefixed octal literal or octal escape sequence. In sloppy 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. 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;

The standardized way to denote octal literals is via the 0o prefix. For example:

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

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. More formally, it's disallowed to have \ followed by any decimal digit other than 0, or \0 followed by a decimal digit; for example \9 and \07.

Setting properties on primitive values

Strict mode forbids setting properties on primitive values. Accessing a property on a primitive implicitly creates a wrapper object that's unobservable, so in sloppy mode, setting properties is ignored (no-op). In strict mode, a TypeError is thrown.

"use strict";

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

Duplicate property names

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.

Removal of the with statement

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.

Non-leaking eval

In strict mode, eval does not introduce new variables into the surrounding scope. In sloppy mode, 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);

Whether the string passed to eval() is evaluated in strict mode depends on how eval() is invoked (direct eval or indirect eval).

Block-scoped function declarations

The JavaScript language specification, since its start, had not allowed function declarations nested in block statements. However, it was so intuitive that most browsers implemented it as an extension grammar. Unfortunately, the implementations' semantics diverged, and it became impossible for the language specification to reconcile all implementations. Therefore, block-scoped function declarations are only explicitly specified in strict mode (whereas they were once disallowed in strict mode), while sloppy mode behavior remains divergent among browsers.

Strict mode makes arguments and eval less bizarrely magical. Both involve a considerable amount of magical behavior in sloppy mode: eval to add or remove bindings and to change binding values, and arguments syncing named arguments with its indexed properties. Strict mode makes great strides toward treating eval and arguments as keywords.

Preventing binding or assigning eval and arguments

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;");

No syncing between parameters and arguments indices

Strict mode code doesn't sync indices of the arguments object with each parameter binding. In a sloppy mode 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);

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.

No this substitution

The value passed as this to a function in strict mode is not forced into being an object (a.k.a. "boxed"). For a sloppy mode function, this is always an object: either the provided object, if called with an object-valued this; or the boxed value of this, if called with a primitive as this; or the global object, if called with undefined or null as 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 is undefined instead of globalThis:

"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)() === true);

Removal of stack-walking properties

In strict mode it's no longer possible to "walk" the JavaScript stack. Many implementations used to implement some extension features that make it possible to detect the upstream caller of a function. 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();

Similarly, arguments.callee is no longer supported. In sloppy mode, 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

Extra reserved words

Reserved words are identifiers that can't be used as variable names. Strict mode reserves some more names than sloppy mode, some of which are already used in the language, and some of which are reserved for the future to make future syntax extensions easier to implement.

• implements
• interface
• let
• package
• private
• protected
• public
• static
• yield

Strict mode has been designed so that the transition to it can be made gradually. It is possible to change each file individually and even to transition code to strict mode down to the function granularity.

You can migrate a codebase to strict mode by first adding "use strict" to a piece of source code, and then fixing all execution errors, while watching out for semantic differences.

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

• Octal syntax const n = 023;
• with statement
• Using delete on a variable name delete myVariable;
• Using eval or arguments as variable or function argument name
• Using one of the newly reserved keywords (in prevision for future language features): implements, interface, let, package, private, protected, public, static, and yield
• Declaring two function parameters with the same name function f(a, b, b) {}
• Declaring the same property name twice in an object literal {a: 1, b: 3, a: 7}. This constraint was later removed (bug 1041128).

These errors are good, because they reveal plain errors or bad practices. They occur before the code is running, so they are easily discoverable as long as the code gets parsed by the runtime.

JavaScript used to silently fail in contexts where what was done should be an error. Strict mode throws in such cases. If your code base contains such cases, testing will be necessary to be sure nothing is broken. You can screen for such errors at the function granularity level.

• Assigning to an undeclared variable throws a ReferenceError. This used to set a property on the global object, which is rarely the expected effect. If you really want to set a value to the global object, explicitly assign it as a property on globalThis.
• Failing to assign to an object's property (e.g. it's read-only) throws a TypeError. In sloppy mode, this would silently fail.
• Deleting a non-deletable property throws a TypeError. In sloppy mode, this would silently fail.
• Accessing arguments.callee, strictFunction.caller, or strictFunction.arguments throws a TypeError if the function is in strict mode. If you are using arguments.callee to call the function recursively, you can use a named function expression instead.

These differences are very subtle differences. It's possible that a test suite doesn't catch this kind of subtle difference. Careful review of your code base will probably be necessary to be sure these differences don't affect the semantics of your code. Fortunately, this careful review can be done gradually down the function granularity.

this

In sloppy mode, function calls like f() would pass the global object as the this value. In strict mode, it is now undefined. When a function was called with call or apply, if the value was a primitive value, this one was boxed into an object (or the global object for undefined and null). In strict mode, the value is passed directly without conversion or replacement.

arguments

In sloppy mode, modifying a value in the arguments object modifies the corresponding named argument. This made optimizations complicated for JavaScript engine and made code harder to read/understand. In strict mode, the arguments object is created and initialized with the same values than the named arguments, but changes to either the arguments object or the named arguments aren't reflected in one another.

eval

In strict mode code, eval doesn't create a new variable in the scope from which it was called. Also, of course, in strict mode, the string is evaluated with strict mode rules. Thorough testing will need to be performed to make sure nothing breaks. Not using eval if you don't really need it may be another pragmatic solution.

Block-scoped function declarations

In sloppy mode, a function declaration inside a block may be visible outside the block and even callable. In strict mode, a function declaration inside a block is only visible inside the block.

• JavaScript modules guide
• Lexical grammar