Indexed collections

This chapter introduces collections of data which are ordered by an index value. This includes arrays and array-like constructs such as Array objects and TypedArray 对象。

Array object

An array is an ordered list of values that you refer to with a name and an index.

For example, consider an array called emp , which contains employees' names indexed by their numerical employee number. So emp[0] would be employee number zero, emp[1] employee number one, and so on.

JavaScript does not have an explicit array data type. However, you can use the predefined Array object and its methods to work with arrays in your applications. The Array object has methods for manipulating arrays in various ways, such as joining, reversing, and sorting them. It has a property for determining the array length and other properties for use with regular expressions.


The following statements create equivalent arrays:

let arr = new Array(element0, element1, ..., elementN)
let arr = Array(element0, element1, ..., elementN)
let arr = [element0, element1, ..., elementN]

element0 , element1 , ..., elementN is a list of values for the array's elements. When these values are specified, the array is initialized with them as the array's elements. The array's length property is set to the number of arguments.

The bracket syntax is called an "array literal" or "array initializer." It's shorter than other forms of array creation, and so is generally preferred. See Array literals 了解细节。

To create an array with non-zero length, but without any items, either of the following can be used:

// This...
let arr = new Array(arrayLength)
// ...results in the same array as this
let arr = Array(arrayLength)
// This has exactly the same effect
let arr = []
arr.length = arrayLength

注意: In the above code, arrayLength 必须是 Number . Otherwise, an array with a single element (the provided value) will be created. Calling arr.length will return arrayLength , but the array doesn't contain any elements. A loop will not find any property on the array.

In addition to a newly defined variable as shown above, arrays can also be assigned as a property of a new or an existing object:

let obj = {}
// ...
obj.prop = [element0, element1, ..., elementN]
// OR
let obj = {prop: [element0, element1, ...., elementN]}

If you wish to initialize an array with a single element, and the element happens to be a Number , you must use the bracket syntax. When a single Number value is passed to the Array() constructor or function, it is interpreted as an arrayLength , not as a single element.

let arr = [42]       // Creates an array with only one element:
                     // the number 42.
let arr = Array(42)  // Creates an array with no elements
                     // and arr.length set to 42.
                     // This is equivalent to:
let arr = []
arr.length = 42

调用 Array( N ) results in a RangeError , if N is a non-whole number whose fractional portion is non-zero. The following example illustrates this behavior.

let arr = Array(9.3)   // RangeError: Invalid array length

If your code needs to create arrays with single elements of an arbitrary data type, it is safer to use array literals. Alternatively, create an empty array first before adding the single element to it.

In ES2015, you can use the Array.of static method to create arrays with single element.

let wisenArray = Array.of(9.3)   // wisenArray contains only one element 9.3

Referring to array elements

Because elements are also properties, you can access the using property accessors . Suppose you define the following array:

let myArray = ['Wind', 'Rain', 'Fire']

You can refer to the first element of the array as myArray[0] , the second element of the array as myArray[1] , etc The index of the elements begins with zero.

注意: You can also use property accessors to access other properties of the array, like with an object.

let arr = ['one', 'two', 'three']
arr[2]          // three
arr['length']   // 3

Populating an array

You can populate an array by assigning values to its elements. For example:

let emp = []
emp[0] = 'Casey Jones'
emp[1] = 'Phil Lesh'
emp[2] = 'August West'

注意: If you supply a non-integer value to the array operator in the code above, a property will be created in the object representing the array, instead of an array element.

let arr = []
arr[3.4] = 'Oranges'
console.log(arr.length)                 // 0
console.log(arr.hasOwnProperty(3.4))    // true

You can also populate an array when you create it:

let myArray = new Array('Hello', myVar, 3.14159)
// OR
let myArray = ['Mango', 'Apple', 'Orange']

Understanding length

At the implementation level, JavaScript's arrays actually store their elements as standard object properties, using the array index as the property name.

length property is special. It always returns the index of the last element plus one. (In the example below, 'Dusty' is indexed at 30 , so cats.length 返回 30 + 1 ).

Remember, JavaScript Array indexes are 0-based: they start at 0 , not 1 . This means that the length property will be one more than the highest index stored in the array:

let cats = []
cats[30] = ['Dusty']
console.log(cats.length) // 31

You can also assign to the length 特性。

Writing a value that is shorter than the number of stored items truncates the array. Writing 0 empties it entirely:

let cats = ['Dusty', 'Misty', 'Twiggy']
console.log(cats.length)  // 3
cats.length = 2
console.log(cats)  // logs "Dusty, Misty" - Twiggy has been removed
cats.length = 0
console.log(cats)  // logs []; the cats array is empty
cats.length = 3
console.log(cats)  // logs [ <3 empty items> ]

Iterating over arrays

A common operation is to iterate over the values of an array, processing each one in some way. The simplest way to do this is as follows:

let colors = ['red', 'green', 'blue']
for (let i = 0; i < colors.length; i++) {

If you know that none of the elements in your array evaluate to false in a boolean context—if your array consists only of DOM nodes, for example—you can use a more efficient idiom:

let divs = document.getElementsByTagName('div')
for (let i = 0, div; div = divs[i]; i++) {
  /* Process div in some way */

This avoids the overhead of checking the length of the array, and ensures that the div variable is reassigned to the current item each time around the loop for added convenience.

forEach() method provides another way of iterating over an array:

let colors = ['red', 'green', 'blue']
colors.forEach(function(color) {
// red
// green
// blue

Alternatively, you can shorten the code for the forEach parameter with ES2015 Arrow Functions:

let colors = ['red', 'green', 'blue']
colors.forEach(color => console.log(color))
// red
// green
// blue

The function passed to forEach is executed once for every item in the array, with the array item passed as the argument to the function. Unassigned values are not iterated in a forEach loop.

Note that the elements of an array that are omitted when the array is defined are not listed when iterating by forEach , but are listed when undefined has been manually assigned to the element:

let array = ['first', 'second', , 'fourth']
array.forEach(function(element) {
// first
// second
// fourth
if (array[2] === undefined) {
  console.log('array[2] is undefined')  // true
array = ['first', 'second', undefined, 'fourth']
array.forEach(function(element) {
// first
// second
// undefined
// fourth

Since JavaScript elements are saved as standard object properties, it is not advisable to iterate through JavaScript arrays using loops, because normal elements and all enumerable properties will be listed.

Array methods

Array object has the following methods:

concat() joins two or more arrays and returns a new array.

let myArray = new Array('1', '2', '3')
myArray = myArray.concat('a', 'b', 'c')
// myArray is now ["1", "2", "3", "a", "b", "c"]

join(delimiter = ',') joins all elements of an array into a string.

let myArray = new Array('Wind', 'Rain', 'Fire')
let list = myArray.join(' - ') // list is "Wind - Rain - Fire"

push() adds one or more elements to the end of an array and returns the resulting length of the array.

let myArray = new Array('1', '2')
myArray.push('3')  // myArray is now ["1", "2", "3"]

pop() removes the last element from an array and returns that element.

let myArray = new Array('1', '2', '3')
let last = myArray.pop()
// myArray is now ["1", "2"], last = "3"

shift() removes the first element from an array and returns that element.

let myArray = new Array('1', '2', '3')
let first = myArray.shift()
// myArray is now ["2", "3"], first is "1"

unshift() adds one or more elements to the front of an array and returns the new length of the array.

let myArray = new Array('1', '2', '3')
myArray.unshift('4', '5')
// myArray becomes ["4", "5", "1", "2", "3"]

slice(start_index, upto_index) extracts a section of an array and returns a new array.

let myArray = new Array('a', 'b', 'c', 'd', 'e')
myArray = myArray.slice(1, 4)  // starts at index 1 and extracts all elements
                               // until index 3, returning [ "b", "c", "d"]

splice(index, count_to_remove, addElement1, addElement2, ...) removes elements from an array and (optionally) replaces them. It returns the items which were removed from the array.

let myArray = new Array('1', '2', '3', '4', '5')
myArray.splice(1, 3, 'a', 'b', 'c', 'd')
// myArray is now ["1", "a", "b", "c", "d", "5"]
// This code started at index one (or where the "2" was),
// removed 3 elements there, and then inserted all consecutive
// elements in its place.

reverse() transposes the elements of an array, in place: the first array element becomes the last and the last becomes the first. It returns a reference to the array.

let myArray = new Array('1', '2', '3')
// transposes the array so that myArray = ["3", "2", "1"]

sort() sorts the elements of an array in place, and returns a reference to the array.

let myArray = new Array('Wind', 'Rain', 'Fire')
// sorts the array so that myArray = ["Fire", "Rain", "Wind"]

sort() can also take a callback function to determine how array elements are compared.

sort method (and others below) that take a callback are known as iterative methods , because they iterate over the entire array in some fashion. Each one takes an optional second argument called thisObject . If provided, thisObject becomes the value of the this keyword inside the body of the callback function. If not provided, as with other cases where a function is invoked outside of an explicit object context, this will refer to the global object ( window ) when using arrow function as callback, or undefined when using normal function as callback.

The callback function is called with two arguments, that are array's elements.

The function below compares two values and returns one of three values:

For instance, the following will sort by the last letter of a string:

let sortFn = function(a, b) {
  if (a[a.length - 1] < b[b.length - 1]) return -1;
  if (a[a.length - 1] > b[b.length - 1]) return 1;
  if (a[a.length - 1] == b[b.length - 1]) return 0;
// sorts the array so that myArray = ["Wind","Fire","Rain"]

indexOf(searchElement[, fromIndex]) searches the array for searchElement and returns the index of the first match.

let a = ['a', 'b', 'a', 'b', 'a']
console.log(a.indexOf('b'))     // logs 1
// Now try again, starting from after the last match
console.log(a.indexOf('b', 2))  // logs 3
console.log(a.indexOf('z'))     // logs -1, because 'z' was not found

lastIndexOf(searchElement[, fromIndex]) works like indexOf , but starts at the end and searches backwards.

let a = ['a', 'b', 'c', 'd', 'a', 'b']
console.log(a.lastIndexOf('b'))     // logs 5
// Now try again, starting from before the last match
console.log(a.lastIndexOf('b', 4))  // logs 1
console.log(a.lastIndexOf('z'))     // logs -1

forEach(callback[, thisObject]) executes callback on every array item and returns undefined .

let a = ['a', 'b', 'c']
a.forEach(function(element) { console.log(element) })
// logs each item in turn

map(callback[, thisObject]) returns a new array of the return value from executing callback on every array item.

let a1 = ['a', 'b', 'c']
let a2 = { return item.toUpperCase() })
console.log(a2) // logs ['A', 'B', 'C']

filter(callback[, thisObject]) returns a new array containing the items for which callback returned true .

let a1 = ['a', 10, 'b', 20, 'c', 30]
let a2 = a1.filter(function(item) { return typeof item === 'number'; })
console.log(a2)  // logs [10, 20, 30]

every(callback[, thisObject]) 返回 true if callback 返回 true for every item in the array.

function isNumber(value) {
  return typeof value === 'number'
let a1 = [1, 2, 3]
console.log(a1.every(isNumber))  // logs true
let a2 = [1, '2', 3]
console.log(a2.every(isNumber))  // logs false

some(callback[, thisObject]) 返回 true if callback 返回 true for at least one item in the array.

function isNumber(value) {
  return typeof value === 'number'
let a1 = [1, 2, 3]
console.log(a1.some(isNumber))  // logs true
let a2 = [1, '2', 3]
console.log(a2.some(isNumber))  // logs true
let a3 = ['1', '2', '3']
console.log(a3.some(isNumber))  // logs false

reduce(callback[, initialValue]) applies callback ( accumulator , currentValue [, currentIndex [, array ]]) for each value in the array for the purpose of reducing the list of items down to a single value.  The reduce function returns the final value returned by callback 函数。

initialValue is specified, then callback is called with initialValue as the first parameter value and the value of the first item in the array as the second parameter value.

initialValue is not specified, then callback 's first two parameter values will be the first and second elements of the array.  On every subsequent call, the first parameter's value will be whatever callback returned on the previous call, and the second parameter's value will be the next value in the array.

callback needs access to the index of the item being processed, on access to the entire array, they are available as optional parameters.

let a = [10, 20, 30]
let total = a.reduce(function(accumulator, currentValue) { return accumulator + currentValue }, 0)
console.log(total) // Prints 60

reduceRight(callback[, initialValue]) works like reduce() , but starts with the last element.

reduce and reduceRight are the least obvious of the iterative array methods. They should be used for algorithms that combine two values recursively in order to reduce a sequence down to a single value.

Multi-dimensional arrays

Arrays can be nested, meaning that an array can contain another array as an element. Using this characteristic of JavaScript arrays, multi-dimensional arrays can be created.

The following code creates a two-dimensional array.

let a = new Array(4)
for (let i = 0; i < 4; i++) {
  a[i] = new Array(4)
  for (let j = 0; j < 4; j++) {
    a[i][j] = '[' + i + ', ' + j + ']'

This example creates an array with the following rows:

Row 0: [0, 0] [0, 1] [0, 2] [0, 3]
Row 1: [1, 0] [1, 1] [1, 2] [1, 3]
Row 2: [2, 0] [2, 1] [2, 2] [2, 3]
Row 3: [3, 0] [3, 1] [3, 2] [3, 3]

Using arrays to store other properties

Arrays can also be used like objects, to store related information.

const arr = [1, 2, 3]; = "value";
console.log(;  // Logs "value"

Arrays and regular expressions

When an array is the result of a match between a regular expression and a string, the array returns properties and elements that provide information about the match. An array is the return value of RegExp.exec() , String.match() ,和 String.split() . For information on using arrays with regular expressions, see 正则表达式 .

Working with array-like objects

Some JavaScript objects, such as the NodeList returned by document.getElementsByTagName() arguments object made available within the body of a function, look and behave like arrays on the surface but do not share all of their methods. The arguments object provides a length attribute but does not implement the forEach() method, for example.

Array methods cannot be called directly on array-like objects.

function printArguments() {
  arguments.forEach(function(item) {  // TypeError: arguments.forEach is not a function

But you can call them indirectly using .

function printArguments() {, function(item) {

Array prototype methods can be used on strings as well, since they provide sequential access to their characters in a similar way to arrays:'a string', function(chr) {


JavaScript 类型数组 are array-like objects and provide a mechanism for accessing raw binary data. As you already know, Array objects grow and shrink dynamically and can have any JavaScript value. JavaScript engines perform optimizations so that these arrays are fast. However, as web applications become more and more powerful, adding features such as audio and video manipulation, access to raw data using WebSockets , and so forth, it has become clear that there are times when it would be helpful for JavaScript code to be able to quickly and easily manipulate raw binary data in typed arrays.


To achieve maximum flexibility and efficiency, JavaScript typed arrays split the implementation into buffers and views . A buffer (implemented by the ArrayBuffer object) is an object representing a chunk of data; it has no format to speak of, and offers no mechanism for accessing its contents. In order to access the memory contained in a buffer, you need to use a view. A view provides a context — that is, a data type, starting offset, and number of elements — that turns the data into an actual typed array.

Typed arrays in an ArrayBuffer


ArrayBuffer is a data type that is used to represent a generic, fixed-length binary data buffer. You can't directly manipulate the contents of an ArrayBuffer ; instead, you create a typed array view or a DataView which represents the buffer in a specific format, and use that to read and write the contents of the buffer.


Typed array views have self descriptive names and provide views for all the usual numeric types like Int8 , Uint32 , Float64 and so forth. There is one special typed array view, {jsxref("Uint8ClampedArray")}}, which clamps the values between 0 and 255 . This is useful for Canvas data processing , for example.

Type 值范围 Size in bytes 描述 Web IDL 类型 Equivalent C type
Int8Array -128 to 127 1 8-bit two's complement signed integer byte int8_t
Uint8Array 0 to 255 1 8-bit unsigned integer octet uint8_t
Uint8ClampedArray 0 to 255 1 8-bit unsigned integer (clamped) octet uint8_t
Int16Array -32768 to 32767 2 16-bit two's complement signed integer short int16_t
Uint16Array 0 to 65535 2 16-bit unsigned integer unsigned short uint16_t
Int32Array -2147483648 to 2147483647 4 32-bit two's complement signed integer long int32_t
Uint32Array 0 to 4294967295 4 32-bit unsigned integer unsigned long uint32_t
Float32Array 1.2 × 10 -38 to 3.4 × 10 38 4 32-bit IEEE floating point number (7 significant digits e.g., 1.1234567 ) unrestricted float float
Float64Array 5.0 × 10 -324 to 1.8 × 10 308 8 64-bit IEEE floating point number (16 significant digits e.g., 1.123...15 ) unrestricted double double
BigInt64Array -2 63 to 2 63 -1 8 64-bit two's complement signed integer bigint int64_t (signed long long)
BigUint64Array 0 to 2 64 -1 8 64-bit unsigned integer bigint uint64_t (unsigned long long)

更多信息,见 JavaScript 类型数组 and the reference documentation for the different TypedArray 对象。

  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. Client-side JavaScript frameworks
    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. Atomics
    6. BigInt
    7. BigInt64Array
    8. BigUint64Array
    9. Boolean
    10. DataView
    11. Date
    12. Error
    13. EvalError
    14. FinalizationRegistry
    15. Float32Array
    16. Float64Array
    17. Function
    18. Generator
    19. GeneratorFunction
    20. Infinity
    21. Int16Array
    22. Int32Array
    23. Int8Array
    24. InternalError
    25. Intl
    26. JSON
    27. Map
    28. Math
    29. NaN
    30. Number
    31. Object
    32. Promise
    33. Proxy
    34. RangeError
    35. ReferenceError
    36. Reflect
    37. RegExp
    38. Set
    39. SharedArrayBuffer
    40. String
    41. Symbol
    42. SyntaxError
    43. TypeError
    44. TypedArray
    45. URIError
    46. Uint16Array
    47. Uint32Array
    48. Uint8Array
    49. Uint8ClampedArray
    50. WeakMap
    51. WeakRef
    52. WeakSet
    53. WebAssembly
    54. decodeURI()
    55. decodeURIComponent()
    56. encodeURI()
    57. encodeURIComponent()
    58. escape()
    59. eval()
    60. globalThis
    61. isFinite()
    62. isNaN()
    63. null
    64. parseFloat()
    65. parseInt()
    66. undefined
    67. unescape()
    68. uneval()
  9. 表达式 & 运算符
    1. Addition (+)
    2. Addition assignment (+=)
    3. Assignment (=)
    4. Bitwise AND (&)
    5. Bitwise AND assignment (&=)
    6. Bitwise NOT (~)
    7. Bitwise OR (|)
    8. Bitwise OR assignment (|=)
    9. Bitwise XOR (^)
    10. Bitwise XOR assignment (^=)
    11. Bitwise operators
    12. Comma operator (,)
    13. 条件 (三元) 运算符
    14. Decrement (--)
    15. Destructuring assignment
    16. Division (/)
    17. Division assignment (/=)
    18. Equality (==)
    19. Exponentiation (**)
    20. Exponentiation assignment (**=)
    21. Function expression
    22. Greater than (>)
    23. Greater than or equal (>=)
    24. Grouping operator ( )
    25. Increment (++)
    26. Inequality (!=)
    27. Left shift assignment (<<=)
    28. Less than (<)
    29. Less than or equal (<=)
    30. Logical AND (&&)
    31. Logical NOT (!)
    32. Logical OR (||)
    33. Multiplication (*)
    34. Multiplication assignment (*=)
    35. Nullish coalescing operator (??)
    36. Object initializer
    37. 运算符优先级
    38. Optional chaining (?.)
    39. Pipeline operator
    40. 特性访问器
    41. Remainder (%)
    42. Remainder assignment (%=)
    43. Right shift assignment (>>=)
    44. 传播句法
    45. Strict equality (===)
    46. Strict inequality (!==)
    47. Subtraction (-)
    48. Subtraction assignment (-=)
    49. Unary negation (-)
    50. Unary plus (+)
    51. Unsigned right shift assignment (>>>=)
    52. 异步函数表达式
    53. await
    54. class expression
    55. delete operator
    56. function* 表达式
    57. in operator
    58. instanceof
    59. new operator
    61. super
    62. this
    63. typeof
    64. void 运算符
    65. yield
    66. yield*
  10. 语句 & 声明
    1. async function
    2. block
    3. break
    4. class
    5. const
    6. continue
    7. debugger
    8. do...while
    9. empty
    10. export
    11. for
    12. for await...of
    14. for...of
    15. 函数声明
    16. function*
    17. if...else
    18. import
    19. import.meta
    20. label
    21. let
    22. return
    23. switch
    24. throw
    25. try...catch
    26. var
    27. while
    28. 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. Private class fields
    2. Public class fields
    3. 构造函数
    4. extends
    5. 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