Transitioning to strict mode

ECMAScript 5 introduced 严格模式 which is now implemented in all major browsers (including IE10). While making web browsers interpret code as strict is easy (just add 'use strict'; at the top of your source code), transitioning an existing code base to strict mode requires a bit more work.

This article aims at providing guidance for developers.

Gradual transition

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.

Differences from non-strict to strict

Syntax errors

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

  • Octal syntax var n = 023;
  • with statement
  • 使用 delete on a variable name delete myVariable ;
  • 使用 eval or arguments as variable or function argument name
  • Using one of the newly reserved keywords (in prevision for ECMAScript 2015): implements , interface , let , package , private , protected , public , static ,和 yield
  • Declaring function in blocks if (a < b) { function f() {} }
  • Obvious errors
    • Declaring twice the same name for a property name in an object literal {a: 1, b: 3, a: 7} This is no longer the case in ECMAScript 2015 ( bug 1041128 ).
    • Declaring two function parameters with the same name function f(a, b, b) {}

These errors are good, because they reveal plain errors or bad practices. They occur before the code is running.

New runtime errors

JavaScript used to silently fail in contexts where what was done was 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. Once again, it can happen at the function granularity level.

Setting a value to an undeclared variable

function f(x) {
  'use strict';
  var a = 12;
  b = a + x * 35; // error!

This used to change a value on the global object which is rarely the expected effect. If you really want to set a value to the global object, pass it as an argument and explicitly assign it as a property:

var global = this; // in the top-level context, "this" always
                   // refers to the global object
function f(x) {
  'use strict';
  var a = 12;
  global.b = a + x * 35;

Trying to delete a non-configurable property

'use strict';
delete Object.prototype; // error!

In non-strict, this would silently fail, in contradiction with the user expectation.

Poisoned arguments and function properties

Accessing arguments.callee , arguments.caller , anyFunction.caller ,或 anyFunction.arguments throws an error in strict mode. The only legitimate use case would be to reuse a function as in:

// example taken from vanillajs:
var s = document.getElementById('thing').style;
s.opacity = 1;
(function() {
  if ((s.opacity-=.1) < 0)
    s.display = 'none';
    setTimeout(arguments.callee, 40);

which can be rewritten as:

'use strict';
var s = document.getElementById('thing').style;
s.opacity = 1;
(function fadeOut() { // name the function
  if((s.opacity-=.1) < 0)
    s.display = 'none';
    setTimeout(fadeOut, 40); // use the name of the function

Semantic differences

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 function calls

In function calls like f() this value was the global object. 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 doesn't alias named function arguments

In non-strict 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.

Change to 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.

Strictness-neutral code

A potential "downside" of moving strict code to strict mode is that the semantics may be different in legacy browsers which do not implement strict mode. In some rare occasions (like bad concatenation or minification), your code also may not run in the mode you wrote and tested it in. Here are the rules to make your code strictness-neutral:

  1. Write your code as strict and make sure no strict-only errors (from the above "New runtime errors" section) are thrown.
  2. Stay away from semantic differences
    1. eval : use it only if you know what you're doing
    2. arguments : always access function arguments via their name or perform a copy of the arguments object using:
      var args =
      as the first line of your function
    3. this : only use this when it refers to an object you created.



  1. JavaScript
  2. 教程:
  3. 完整初学者
    1. JavaScript 基础
    2. JavaScript 第一步
    3. JavaScript 构建块
    4. 引入 JavaScript 对象
  4. JavaScript 指南
    1. 介绍
    2. 语法和类型
    3. 控制流程和错误处理
    4. 循环和迭代
    5. 函数
    6. 表达式和运算符
    7. 数字和日期
    8. 文本格式
    9. 正则表达式
    10. Indexed collections
    11. Keyed collections
    12. Working with objects
    13. 对象模型的细节
    14. Using promises
    15. 迭代器和生成器
    16. Meta programming
    17. JavaScript 模块
  5. 中间体
    1. 引入 JavaScript 对象
    2. 客户端侧 Web API
    3. 重新介绍 JavaScript
    4. JavaScript 数据结构
    5. 相等比较和相同
    6. 闭包
  6. 高级
    1. 继承和原型链
    2. 严格模式
    3. JavaScript 类型数组
    4. 内存管理
    5. 并发模型和事件循环
  7. 参考:
  8. 内置对象
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncFunction
    5. AsyncIterator
    6. Atomics
    7. BigInt
    8. BigInt64Array
    9. BigUint64Array
    10. Boolean
    11. DataView
    12. Date
    13. Error
    14. EvalError
    15. Float32Array
    16. Float64Array
    17. Function
    18. Generator
    19. GeneratorFunction
    20. Infinity
    21. Int16Array
    22. Int32Array
    23. Int8Array
    24. InternalError
    25. Intl
    26. Intl.Collator
    27. Intl.DateTimeFormat
    28. Intl.DisplayNames
    29. Intl.ListFormat
    30. Intl.Locale
    31. Intl.NumberFormat
    32. Intl.PluralRules
    33. Intl.RelativeTimeFormat
    34. Iterator
    35. JSON
    36. Map
    37. Math
    38. NaN
    39. Number
    40. Object
    41. Promise
    42. Proxy
    43. RangeError
    44. ReferenceError
    45. Reflect
    46. RegExp
    47. Set
    48. SharedArrayBuffer
    49. String
    50. Symbol
    51. SyntaxError
    52. TypeError
    53. TypedArray
    54. URIError
    55. Uint16Array
    56. Uint32Array
    57. Uint8Array
    58. Uint8ClampedArray
    59. WeakMap
    60. WeakSet
    61. WebAssembly
    62. decodeURI()
    63. decodeURIComponent()
    64. encodeURI()
    65. encodeURIComponent()
    66. escape()
    67. eval()
    68. globalThis
    69. isFinite()
    70. isNaN()
    71. null
    72. parseFloat()
    73. parseInt()
    74. undefined
    75. unescape()
    76. uneval()
  9. 表达式 & 运算符
    1. 算术运算符
    2. Array comprehensions
    3. 赋值运算符
    4. Bitwise operators
    5. 逗号运算符
    6. Comparison operators
    7. 条件 (三元) 运算符
    8. Destructuring assignment
    9. Expression closures
    10. Function expression
    11. Generator comprehensions
    12. Grouping operator
    13. Legacy generator function expression
    14. Logical operators
    15. Nullish coalescing operator
    16. Object initializer
    17. 运算符优先级
    18. Optional chaining
    19. Pipeline operator
    20. 特性访问器
    21. 传播句法
    22. 异步函数表达式
    23. await
    24. class expression
    25. delete operator
    26. function* 表达式
    27. in operator
    28. instanceof
    29. new operator
    31. super
    32. this
    33. typeof
    34. void 运算符
    35. yield
    36. yield*
  10. 语句 & 声明
    1. Legacy generator function
    2. async function
    3. block
    4. break
    5. class
    6. const
    7. continue
    8. debugger
    9. default
    10. do...while
    11. empty
    12. export
    13. for
    14. for await...of
    15. for
    17. for...of
    18. 函数声明
    19. function*
    20. if...else
    21. import
    22. import.meta
    23. label
    24. let
    25. return
    26. switch
    27. throw
    28. try...catch
    29. var
    30. while
    31. with
  11. 函数
    1. 箭头函数表达式
    2. 默认参数
    3. 方法定义
    4. 其余参数
    5. 自变量对象
    6. getter
    7. setter
    1. Class fields
    2. 构造函数
    3. extends
    4. static
  12. 错误
    1. Error: Permission denied to access property "x"
    2. InternalError: too much recursion
    3. RangeError: argument is not a valid code point
    4. RangeError: invalid array length
    5. RangeError: invalid date
    6. RangeError: precision is out of range
    7. RangeError: radix must be an integer
    8. RangeError: repeat count must be less than infinity
    9. RangeError: repeat count must be non-negative
    10. ReferenceError: "x" is not defined
    11. ReferenceError: assignment to undeclared variable "x"
    12. ReferenceError: can't access lexical declaration`X' before initialization
    13. ReferenceError: deprecated caller or arguments usage
    14. ReferenceError: invalid assignment left-hand side
    15. ReferenceError: reference to undefined property "x"
    16. SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
    17. SyntaxError: "use strict" not allowed in function with non-simple parameters
    18. SyntaxError: "x" is a reserved identifier
    19. SyntaxError: JSON.parse: bad parsing
    20. SyntaxError: Malformed formal parameter
    21. SyntaxError: Unexpected token
    22. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    23. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    24. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
    25. SyntaxError: for-in loop head declarations may not have initializers
    26. SyntaxError: function statement requires a name
    27. SyntaxError: identifier starts immediately after numeric literal
    28. SyntaxError: illegal character
    29. SyntaxError: invalid regular expression flag "x"
    30. SyntaxError: missing ) after argument list
    31. SyntaxError: missing ) after condition
    32. SyntaxError: missing : after property id
    33. SyntaxError: missing ; before statement
    34. SyntaxError: missing = in const declaration
    35. SyntaxError: missing ] after element list
    36. SyntaxError: missing formal parameter
    37. SyntaxError: missing name after . operator
    38. SyntaxError: missing variable name
    39. SyntaxError: missing } after function body
    40. SyntaxError: missing } after property list
    41. SyntaxError: redeclaration of formal parameter "x"
    42. SyntaxError: return not in function
    43. SyntaxError: test for equality (==) mistyped as assignment (=)?
    44. SyntaxError: unterminated string literal
    45. TypeError: "x" has no properties
    46. TypeError: "x" is (not) "y"
    47. TypeError: "x" is not a constructor
    48. TypeError: "x" is not a function
    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
    63. TypeError: invalid 'instanceof' operand 'x'
    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
    73. Warning: Date.prototype.toLocaleFormat is deprecated
    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
    1. JavaScript technologies overview
    2. 词汇语法
    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征