Quick Reference for Changes in Strict Mode

The following is a quick reference containing a list of changes made in JavaScript's strict mode. For more information on how and when strict mode is enabled, refer to MDN's page on strict mode. That page also includes much (but not all) that's listed below in greater detail.

Reserved words

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

// sloppy: Variables created
// strict: Error

Assign undeclared

doesNotExist = 0

// sloppy: Creates global property
// strict: Error

Assign built-in globals

undefined = 0
Infinity = 0
NaN = 0

// sloppy: Silently fails assignment
// strict: Error

Assign getter

const hasGetter = {
  get getter() {}
}
hasGetter.getter = 0

// sloppy: Silently fails assignment
// strict: Error

Assign non-writable

const hasReadOnly = {}
Object.defineProperty(hasReadOnly, "readOnly", { value: 1, writable: false })
hasReadOnly.readOnly = 0

// sloppy: Silently fails assignment
// strict: Error

Assign to non-extensible

const nonExtensible = Object.preventExtensions({})
nonExtensible.any = 0

// sloppy: Silently fails assignment
// strict: Error

Assign primitive properties

"string".prop = 0
true.prop = 0
0.0.prop = 0

// sloppy: Silently fails assignment
// strict: Error

Delete non-configurable

const hasNonConfig = {}
Object.defineProperty(hasNonConfig, "nonConfig", { value: 1, configurable: false })
delete hasNonConfig.nonConfig

// sloppy: Does not delete, evaluates to false
// strict: Error

Delete unqualified identifier

delete identifier

// sloppy: Deletes and evaluates to true if identifier is a deletable global,
//    otherwise does not delete and evaluates to false
// strict: Error

Delete from proxy with trap returning falsy

const hasDeleteTrap = new Proxy({}, {
  deleteProperty() {
    return false
  }
})
delete hasDeleteTrap.any

// sloppy: No error
// strict: Error

with blocks

with({}) {}

// sloppy: No error
// strict: Error

Initializers in for-in variable declarations

for (var x = 0 in {}) {}

// sloppy: No error, identifier will be assigned to initializer value if no iterations
// strict: Error

0-prefixed numeric literals

01
08

// sloppy: Evaluates to octal value if digit after 0 < 8, decimal if > 7 
// strict: Error

Octal escapes in string literals

"\7"

// sloppy: Creates string '\x07'
// strict: Error

No in-scope declarations with eval

eval("var fromEval = 0")

// sloppy: Declaration in eval added to current scope
// strict: Declaration does not get added to current scope

eval rebinding

eval = 0
let eval
function eval() {}
function fn(eval) {}

// sloppy: Rebinding succeeds
// strict: Error

Assign function expression name

const fn = function name() {
  name = 0
}

// sloppy: No error
// strict: Error

Labeled function declarations

label: function fn() {}

// sloppy: No error
// strict: Error

Block-scope function declarations

function outer() {
  {
    function inner() {}
  }
  inner()
}

// sloppy: Inner function scoped to outer function and can be called outside of block
// strict: Inner function scoped to block and cannot be called outside of block

Duplicate parameter names

function fn(a, a) {}

// sloppy: No error, last parameter of that name used in function body
// strict: Error

Function caller, arguments properties

function fn() {
  fn.caller
  fn.arguments
}

// sloppy: No error
// strict: Error

Arguments callee property

function fn() {
  arguments.callee
}

// sloppy: No error
// strict: Error

Parameters mapped to arguments

function assignArg(param) {
  arguments[0] = 1;
  param = 2;
}

// sloppy: Assignment to arguments changes parameter value, assigning parameter changes arguments
// strict: Assignments do not change other values

arguments rebinding

arguments = 0
let arguments
function arguments() {}
function fn(arguments) {}

// sloppy: Rebinding succeeds
// strict: Error

this in function calls

function fn() {
  return this
}

// sloppy: Function sees `this` as global object
// strict: Function sees `this` as undefined

Nullish primitive this in function calls

function fn() {
  return this
}
fn.call(null)
fn.call(undefined)

// sloppy: Function sees `this` as global object
// strict: Function sees `this` as original primitive

Non-nullish primitive this in function calls

function fn() {
  return this
}
fn.call("string")
fn.call(true)
fn.call(0)

// sloppy: Function sees `this` as `Object(this)`
// strict: Function sees `this` as original primitive

Appendix

Changes in modules

Modules are always in strict mode, but there are some additional changes that are specific to modules that do not apply to strict mode in other contexts.

Reserved words

let await

// scripts: Variable created
// modules: Error

Duplicate function declarations

function fn() {}
function fn() {}

// scripts: Last declaration assigned to identifier
// modules: Error

Changes with parameters

Features used within parameter lists can also impact how JavaScript behaves with some strict mode-like behaviors observable in sloppy mode depending on how a parameter list is defined. These changes are seen when parameter lists contains any advanced features such as default parameters, rest parameters, or destructured parameters.

Duplicate parameter names

function simple(a, a) {}
function nonSimple(a, a = 0) {}

// simple parameter list: No error, last parameter of that name used in function body
// non-simple parameter list: Error

"use strict" directive

function simple(a) {
  "use strict"
}
function nonSimple(a = 0) {
  "use strict"
}

// simple parameter list: No error
// non-simple parameter list: Error

Parameters mapped to arguments

function simple(param) {
  arguments[0] = 1;
  param = 2;
}
function nonSimple(param = 0) {
  arguments[0] = 1;
  param = 2;
}

// simple parameter list: Assignment to arguments changes parameter value, assigning parameter changes arguments
// non-simple parameter list: Assignments do not change other values