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Why Do We Need Strict Mode

by Mozilla ContributorsDecember 16th, 2020

Too Long; Didn't Read

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 makes several changes to normal JavaScript semantics. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do not support it. The entire contents of JavaScript modules are automatically in strict mode, with no statement needed to initiate it. To invoke strict mode for an entire script, put the exact statement "use strict" before any other statements.

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JavaScript's strict mode, introduced in ECMAScript 5, 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:

Eliminates some JavaScript silent errors by changing them to throw errors.
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.
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.

Invoking strict mode

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

WindowTimers.setTimeout()

, and related functions are entire scripts, and invoking strict mode in them works as expected.

Strict mode for scripts

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

This syntax has a trap that has already bitten a major site: it isn't possible to blindly concatenate conflicting scripts. Consider concatenating a strict mode script with a non-strict mode script: the entire concatenation looks strict! The inverse is also true: non-strict plus strict looks non-strict. Obviously, concatenation of scripts is never ideal, but if you must, consider enabling strict on a function-by-function basis.

You can also take the approach of wrapping the entire contents of a script in a function and having that outer function use strict mode. This eliminates the concatenation problem and it means that you have to explicitly export any shared variables out of the function scope.

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.

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."; }

Strict mode for modules

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

Changes in strict mode

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.

Converting mistakes into errors

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';
                       // Assuming no global variable mistypedVariable exists
mistypeVariable = 17;  // this line throws a ReferenceError due to the 
                       // misspelling of variable

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
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 = {};
Object.preventExtensions(fixed);
fixed.newProp = 'ohai'; // throws a TypeError

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

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

Fourth, strict mode prior to Gecko 34 requires that all properties named in an object literal be unique. The normal code may duplicate property names, with the last one determining the property's value. But since only the last one does anything, the duplication is simply a vector for bugs, if the code is modified to change the property value other than by changing the last instance. Duplicate property names are a syntax error in strict mode:

This is no longer the case in ECMAScript 2015 (bug 1041128).

'use strict';
var o = { p: 1, p: 2 }; // !!! syntax error

Fifth, 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
}

Sixth, a strict mode in ECMAScript 5 forbids octal syntax. The octal syntax isn't part of ECMAScript 5, but it's supported in all browsers by prefixing the octal number with a zero:

0644 === 420

and

"\045" === "%"

. In ECMAScript 2015 Octal number is supported by prefixing a number with "

0o

". i.e.

var a = 0o10; // ES2015: Octal

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

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

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

Seventh, strict mode in ECMAScript 2015 forbids setting properties on primitive values. Without strict mode, setting properties is simply ignored (no-op), with strict mode, however, a

TypeError

is thrown.

(function() {
'use strict';

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

})();

Simplifying variable uses

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 ptimizing 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';
var x = 17;
with (obj) { // !!! syntax error
  // If this weren't strict mode, would this be var 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

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

Making

eval

and

arguments

simpler

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;
var obj = { set p(arguments) { } };
var eval;
try { } catch (arguments) { }
function x(eval) { }
function arguments() { }
var y = function eval() { };
var 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]];
}
var 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: simply 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';
var f = function() { return arguments.callee; };
f(); // throws a TypeError

"Securing" JavaScript

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

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)() === true);

That means, among other things, that in browsers it's no longer possible to reference the

window

object through

this

inside a strict mode function.

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();

Third,

arguments

for strict mode functions no longer provide access to the corresponding function call's variables. In some old ECMAScript implementations

arguments.caller

was an object whose properties aliased variables in that function. This is a security hazard because it breaks the ability to hide privileged values via function abstraction; it also precludes most optimizations.

For these reasons no recent browsers implement it. Yet because of its historical functionality,

arguments.caller

for a strict mode function is also a non-deletable property which throws when set or retrieved:

'use strict';
function fun(a, b) {
  'use strict';
  var v = 12;
  return arguments.caller; // throws a TypeError
}
fun(1, 2); // doesn't expose v (or a or b)

Paving the way for future ECMAScript versions

Future ECMAScript versions will likely introduce new syntax, and strict mode in ECMAScript 5 applies some restrictions to ease the transition. It will be easier to make some changes if the foundations of those changes are prohibited in strict mode.

First, in strict mode, a short list of identifiers become reserved keywords. These words are

implements

interface

let

package

private

protected

public

static

, and

yield

. In strict mode, then, you can't name or use variables or arguments with these names.

function package(protected) { // !!!
  'use strict';
  var implements; // !!!

  interface: // !!!
  while (true) {
    break interface; // !!!
  }

  function private() { } // !!!
}
function fun(static) { 'use strict'; } // !!!

Two Mozilla-specific caveats: First, if your code is JavaScript 1.7 or greater (for example in chrome code or when using the right

<script type="">

) and is strict mode code,

let

and

yield

have the functionality they've had since those keywords were first introduced.

But strict mode code on the web, loaded with

<script src="">

<script>...</script>

, won't be able to use

let/yield

as identifiers. Second, while ES5 unconditionally reserves the words

class

enum

export

extends

import

, and

super

, before Firefox 5 Mozilla reserved them only in strict mode.

Second, strict mode prohibits function statements, 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.

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

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

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

This prohibition isn't strict mode proper because such function statements are an extension of basic ES5. But it is the recommendation of the ECMAScript committee, and browsers will implement it.

Strict mode in browsers

The major browsers now implement strict mode. However, don't blindly depend on it since there still are numerous Browser versions used in the wild that only have partial support for strict mode or do not support it at all (e.g. Internet Explorer below version 10!). Strict mode changes semantics. Relying on those changes will cause mistakes and errors in browsers which don't implement strict mode.

Exercise caution in using strict mode, and back up reliance on strict mode with feature tests that check whether relevant parts of strict mode are implemented. Finally, make sure to test your code in browsers that do and don't support strict mode. If you test only in browsers that don't support strict mode, you're very likely to have problems in browsers that do, and vice versa.