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Array TypedArray

emp emp[0] 0emp[1] 1

JavaScript Array Array

js
const arr1 = new Array(element0, element1, /* , */ elementN);
const arr2 = Array(element0, element1, /* , */ elementN);
const arr3 = [element0, element1, /* , */ elementN];

element0, element1, , elementN length

js
// ...
const arr1 = new Array(arrayLength);

// ...
const arr2 = Array(arrayLength);

// 
const arr3 = [];
arr3.length = arrayLength;

: arrayLength Numberarr.length arrayLength for...in

js
const obj = {};
// 
obj.prop = [element0, element1, /* , */ elementN];

// 
const obj = { prop: [element0, element1, /* , */ elementN] };

Number Number Array() arrayLength

42

js
const arr = [42];

arr.length 42

js
const arr = Array(42);

js
const arr = [];
arr.length = 42;

N Array(N) RangeError

js
const arr = Array(9.3); // RangeError: Invalid array length

ES2015 Array.of

js
const arr = Array.of(9.3); // arr  1  9.3

js
const myArray = ["Wind", "Rain", "Fire"];

0 1 myArray[0]2 myArray[1]

:

js
const arr = ["one", "two", "three"];
arr[2]; // three
arr["length"]; // 3

js
const emp = [];
emp[0] = "Casey Jones";
emp[1] = "Phil Lesh";
emp[2] = "August West";

:

js
const arr = [];
arr[3.4] = "Oranges";
console.log(arr.length); // 0
console.log(Object.hasOwn(arr, 3.4)); // true

js
const myArray = new Array("Hello", myVar, 3.14159);
// 
const myArray = ["Mango", "Apple", "Orange"];

JavaScript

length 'Dusty' 30 cats.length 30 + 1

JavaScript 0 1 length 1

js
const cats = [];
cats[30] = ["Dusty"];
console.log(cats.length); // 31

length

0

js
const cats = ["Dusty", "Misty", "Twiggy"];
console.log(cats.length); // 3

cats.length = 2;
console.log(cats); // [ 'Dusty', 'Misty' ] - Twiggy 

cats.length = 0;
console.log(cats); // [];  cats 

cats.length = 3;
console.log(cats); // [ <3 > ]

js
const colors = ["red", "green", "blue"];
for (let i = 0; i < colors.length; i++) {
  console.log(colors[i]);
}

false DOM

js
const divs = document.getElementsByTagName("div");
for (let i = 0, div; (div = divs[i]); i++) {
  /* div  */
}

div

forEach()

js
const colors = ["red", "green", "blue"];
colors.forEach((color) => console.log(color));
// red
// green
// blue

forEach 1 forEach

forEach undefined

js
const sparseArray = ["first", "second", , "fourth"];

sparseArray.forEach((element) => {
  console.log(element);
});
// Logs:
// first
// second
// fourth

if (sparseArray[2] === undefined) {
  console.log("sparseArray[2] is undefined"); // true
}

const nonsparseArray = ["first", "second", undefined, "fourth"];

nonsparseArray.forEach((element) => {
  console.log(element);
});
// Logs:
// first
// second
// undefined
// fourth

JavaScript for...in JavaScript

Array

concat() 2

js
let myArray = ["1", "2", "3"];
myArray = myArray.concat("a", "b", "c");
// myArray  ["1", "2", "3", "a", "b", "c"]

join()

js
const myArray = ["Wind", "Rain", "Fire"];
const list = myArray.join(" - "); // list  "Wind - Rain - Fire"

push() 1

js
const myArray = ["1", "2"];
myArray.push("3"); // myArray  ["1", "2", "3"]

pop()

js
const myArray = ["1", "2", "3"];
const last = myArray.pop();
// myArray  ["1", "2"] last  "3"

shift()

js
const myArray = ["1", "2", "3"];
const first = myArray.shift();
// myArray  ["2", "3"]first  "1"

unshift() 1

js
const myArray = ["1", "2", "3"];
myArray.unshift("4", "5");
// myArray  ["4", "5", "1", "2", "3"]

slice()

js
let myArray = ["a", "b", "c", "d", "e"];
myArray = myArray.slice(1, 4); // [ "b", "c", "d"]
//  1  3 
//

at() undefined

js
const myArray = ["a", "b", "c", "d", "e"];
myArray.at(-2); // "d", the second-last element of myArray

splice() ()

js
const myArray = ["1", "2", "3", "4", "5"];
myArray.splice(1, 3, "a", "b", "c", "d");
// myArray  ["1", "a", "b", "c", "d", "5"] 
//  1 "2" 
// 3

reverse()

js
const myArray = ["1", "2", "3"];
myArray.reverse();
// myArray = ["3", "2", "1"]

flat()

js
let myArray = [1, 2, [3, 4]];
myArray = myArray.flat();
// myArray  [1, 2, 3, 4]  [3, 4]

sort()

js
const myArray = ["Wind", "Rain", "Fire"];
myArray.sort();
// myArray = ["Fire", "Rain", "Wind"]

sort() 2 2 2 0

js
const sortFn = (a, b) => {
  if (a[a.length - 1] < b[b.length - 1]) {
    return -1; //  => a < ba  b 
  } else if (a[a.length - 1] > b[b.length - 1]) {
    return 1; //  => a > ba  b 
  }
  return 0; //  => a = ba  b 
};
myArray.sort(sortFn);
//  myArray = ["Wind","Fire","Rain"]
  • a b -1
  • a b 1
  • a b 0

indexOf() searchElement

js
const a = ["a", "b", "a", "b", "a"];
console.log(a.indexOf("b")); // 1 

// 
console.log(a.indexOf("b", 2)); // 3
console.log(a.indexOf("z")); // -1'z'

lastIndexOf() indexOf

js
const a = ["a", "b", "c", "d", "a", "b"];
console.log(a.lastIndexOf("b")); // 5

// 
console.log(a.lastIndexOf("b", 4)); // 1
console.log(a.lastIndexOf("z")); // -1

forEach() callback undefined

js
const a = ["a", "b", "c"];
a.forEach((element) => {
  console.log(element);
});
// :
// a
// b
// c

forEach thisArg 2 thisArg this this windowglobalThis undefined

: sort() sort() thisArg

map() callback

js
const a1 = ["a", "b", "c"];
const a2 = a1.map((item) => item.toUpperCase());
console.log(a2); // ['A', 'B', 'C']

flatMap() map() 1 flat()

js
const a1 = ["a", "b", "c"];
const a2 = a1.flatMap((item) => [item.toUpperCase(), item.toLowerCase()]);
console.log(a2); // ['A', 'a', 'B', 'b', 'C', 'c']

filter() callback true

js
const a1 = ["a", 10, "b", 20, "c", 30];
const a2 = a1.filter((item) => typeof item === "number");
console.log(a2); // [10, 20, 30]

find() callback true

js
const a1 = ["a", 10, "b", 20, "c", 30];
const i = a1.find((item) => typeof item === "number");
console.log(i); // 10

findLast() callback true

js
const a1 = ["a", 10, "b", 20, "c", 30];
const i = a1.findLast((item) => typeof item === "number");
console.log(i); // 30

findIndex() callback true

js
const a1 = ["a", 10, "b", 20, "c", 30];
const i = a1.findIndex((item) => typeof item === "number");
console.log(i); // 1

findLastIndex() callback true

js
const a1 = ["a", 10, "b", 20, "c", 30];
const i = a1.findLastIndex((item) => typeof item === "number");
console.log(i); // 5

every() callback true true

js
function isNumber(value) {
  return typeof value === "number";
}
const a1 = [1, 2, 3];
console.log(a1.every(isNumber)); // true
const a2 = [1, "2", 3];
console.log(a2.every(isNumber)); // false

some() callback true true

js
function isNumber(value) {
  return typeof value === "number";
}
const a1 = [1, 2, 3];
console.log(a1.some(isNumber)); // true
const a2 = [1, "2", 3];
console.log(a2.some(isNumber)); // true
const a3 = ["1", "2", "3"];
console.log(a3.some(isNumber)); // false

reduce() callback(accumulator, currentValue, currentIndex, array) 1 reduce callback

initialValue initialValue 1 2 callback

initialValue callback 2 2 callback 2

callback

js
const a = [10, 20, 30];
const total = a.reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  0,
);
console.log(total); // 60

reduceRight() reduce()

reduce reduceRight 2

Object.groupBy()

name type "food"

js
const inventory = [
  { name: "asparagus", type: "vegetables" },
  { name: "bananas", type: "fruit" },
  { name: "goat", type: "meat" },
  { name: "cherries", type: "fruit" },
  { name: "fish", type: "meat" },
];

Object.groupBy()

js
const result = Object.groupBy(inventory, ({ type }) => type);
console.log(result);
// Logs
// {
//   vegetables: [{ name: 'asparagus', type: 'vegetables' }],
//   fruit: [
//     { name: 'bananas', type: 'fruit' },
//     { name: 'cherries', type: 'fruit' }
//   ],
//   meat: [
//     { name: 'goat', type: 'meat' },
//     { name: 'fish', type: 'meat' }
//   ]
// }

Map.groupBy() Map Object.groupBy()

undefined

js
// Array :
const a = Array(5); // [ <5 > ]

// :
const b = [1, 2, , , 5]; // [ 1, 2, <2 >, 5 ]

// array.length :
const c = [1, 2];
c[4] = 5; // [ 1, 2, <2 >, 5 ]

// .length :
const d = [1, 2];
d.length = 5; // [ 1, 2, <3 > ]

// :
const e = [1, 2, 3, 4, 5];
delete e[2]; // [ 1, 2, <1 >, 4, 5 ]

undefined

js
const arr = [1, 2, , , 5]; // 

// 
console.log(arr[2]); // undefined

// For...of
for (const i of arr) {
  console.log(i);
}
// : 1 2 undefined undefined 5

// 
const another = [...arr]; // "another"  [ 1, 2, undefined, undefined, 5 ]

js
const mapped = arr.map((i) => i + 1); // [ 2, 3, <2 >, 6 ]
arr.forEach((i) => console.log(i)); // 1 2 5
const filtered = arr.filter(() => true); // [ 1, 2, 5 ]
const hasFalsy = arr.some((k) => !k); // false

// Property enumeration
const keys = Object.keys(arr); // [ '0', '1', '4' ]
for (const key in arr) {
  console.log(key);
}
// : '0' '1' '4'
// 
const objectSpread = { ...arr }; // { '0': 1, '1': 2, '4': 5 }

Array

JavaScript

2

js
const 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}]`;
  }
}


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]

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

RegExp.prototype.exec(), String.prototype. match(), String.prototype.split()

document.getElementsByTagName() NodeList arguments JavaScript arguments length forEach()

js
function printArguments() {
  arguments.forEach((item) => {
    console.log(item);
  }); // TypeError: arguments.forEach is not a function
}

Function.prototype.call()

js
function printArguments() {
  Array.prototype.forEach.call(arguments, (item) => {
    console.log(item);
  });
}

js
Array.prototype.forEach.call("a string", (chr) => {
  console.log(chr);
});

MDN

MDN 2026412

GitHub
  1. JavaScript
  3. JavaScript
    15. JavaScript
    19. JavaScript
    2. JavaScript
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncDisposableStack
    5. AsyncFunction
    6. AsyncGenerator
    7. AsyncGeneratorFunction
    8. AsyncIterator
    9. Atomics
    10. BigInt
    11. BigInt64Array
    12. BigUint64Array
    13. Boolean
    14. DataView
    15. Date
    16. decodeURI()
    17. decodeURIComponent()
    18. DisposableStack
    19. encodeURI()
    20. encodeURIComponent()
    21. Error
    22. escape()
    23. eval()
    24. EvalError
    25. FinalizationRegistry
    26. Float16Array
    27. Float32Array
    28. Float64Array
    29. Function
    30. Generator
    31. GeneratorFunction
    32. globalThis
    33. Infinity
    34. Int8Array
    35. Int16Array
    36. Int32Array
    37. InternalError
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    40. isNaN()
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    48. parseFloat()
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    60. Symbol
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    65. Uint8Array
    66. Uint8ClampedArray
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    69. undefined
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    29. ( )
    30. import.meta
      1. import.meta.resolve()
    31. import()
    32. in
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    35. instanceof
    36. (<<)
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    38. (<)
    39. (<=)
    40. (&&)
    41. (&&=)
    42. (!)
    43. (||)
    44. (||=)
    45. (*)
    46. (*=)
    47. new
    48. new.target
    49. null
    50. (??=)
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    56. (%)
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    58. (>>)
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    5. static
    1. : \1, \2
    2. : (...)
    3. : \d, \D, \w, \W, \s, \S
    4. : [...], [^...]
    5. : \n, \u{...}
    6. : |
    7. : ^, $
    8. : a, b
    9. : (?=...), (?!...)
    10. : (?<=...), (?<!...)
    11. : (?ims-ims:...)
    12. : \k<name>
    13. : (?<name>...)
    14. : (?:...)
    15. : *, +, ?, {n}, {n,}, {n,m}
    16. Unicode : \p{...}, \P{...}
    17. : .
    18. : \b, \B
    1. AggregateError: No Promise in Promise.any was resolved
    2. Error: Permission denied to access property "x"
    3. InternalError: too much recursion
    4. RangeError: argument is not a valid code point
    5. RangeError: BigInt division by zero
    6. RangeError: BigInt negative exponent
    7. RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'
    8. RangeError: invalid array length
    9. RangeError: invalid date
    10. RangeError: precision is out of range
    11. RangeError: radix must be an integer
    12. RangeError: repeat count must be less than infinity
    13. RangeError: repeat count must be non-negative
    14. RangeError: x can't be converted to BigInt because it isn't an integer
    15. ReferenceError: "x" is not defined
    16. ReferenceError: assignment to undeclared variable "x"
    17. ReferenceError: can't access lexical declaration`X' before initialization
    18. ReferenceError: must call super constructor before using 'this' in derived class constructor
    19. ReferenceError: super() called twice in derived class constructor
    20. SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code
    21. SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
    22. SyntaxError: "use strict" not allowed in function with non-simple parameters
    23. SyntaxError: "x" is a reserved identifier
    24. SyntaxError: \ at end of pattern
    25. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    26. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
    27. SyntaxError: arguments is not valid in fields
    28. SyntaxError: await is only valid in async functions, async generators and modules
    29. SyntaxError: await/yield expression can't be used in parameter
    30. SyntaxError: cannot use ?? unparenthesized within || and && expressions
    31. SyntaxError: character class escape cannot be used in class range in regular expression
    32. SyntaxError: continue must be inside loop
    33. SyntaxError: duplicate capture group name in regular expression
    34. SyntaxError: duplicate formal argument x
    35. SyntaxError: for-in loop head declarations may not have initializers
    36. SyntaxError: function statement requires a name
    37. SyntaxError: functions cannot be labelled
    38. SyntaxError: getter and setter for private name #x should either be both static or non-static
    39. SyntaxError: getter functions must have no arguments
    40. SyntaxError: identifier starts immediately after numeric literal
    41. SyntaxError: illegal character
    42. SyntaxError: import declarations may only appear at top level of a module
    43. SyntaxError: incomplete quantifier in regular expression
    44. SyntaxError: invalid assignment left-hand side
    45. SyntaxError: invalid BigInt syntax
    46. SyntaxError: invalid capture group name in regular expression
    47. SyntaxError: invalid character in class in regular expression
    48. SyntaxError: invalid class set operation in regular expression
    49. SyntaxError: invalid decimal escape in regular expression
    50. SyntaxError: invalid identity escape in regular expression
    51. SyntaxError: invalid named capture reference in regular expression
    52. SyntaxError: invalid property name in regular expression
    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
    55. SyntaxError: invalid regular expression flag "x"
    56. SyntaxError: invalid unicode escape in regular expression
    57. SyntaxError: JSON.parse: bad parsing
    58. SyntaxError: label not found
    59. SyntaxError: missing : after property id
    60. SyntaxError: missing ) after argument list
    61. SyntaxError: missing ) after condition
    62. SyntaxError: missing ] after element list
    63. SyntaxError: missing } after function body
    64. SyntaxError: missing } after property list
    65. SyntaxError: missing = in const declaration
    66. SyntaxError: missing formal parameter
    67. SyntaxError: missing name after . operator
    68. SyntaxError: missing variable name
    69. SyntaxError: negated character class with strings in regular expression
    70. SyntaxError: new keyword cannot be used with an optional chain
    71. SyntaxError: nothing to repeat
    72. SyntaxError: numbers out of order in {} quantifier.
    73. SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code
    74. SyntaxError: parameter after rest parameter
    75. SyntaxError: private fields can't be deleted
    76. SyntaxError: property name __proto__ appears more than once in object literal
    77. SyntaxError: raw bracket is not allowed in regular expression with unicode flag
    78. SyntaxError: redeclaration of formal parameter "x"
    79. SyntaxError: reference to undeclared private field or method #x
    80. SyntaxError: rest parameter may not have a default
    81. SyntaxError: return not in function
    82. SyntaxError: setter functions must have one argument
    83. SyntaxError: unterminated string literal
    84. SyntaxError: super() is only valid in derived class constructors
    85. SyntaxError: tagged template cannot be used with optional chain
    86. SyntaxError: Unexpected '#' used outside of class body
    87. SyntaxError: Unexpected token
    88. SyntaxError: unlabeled break must be inside loop or switch
    89. SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'
    90. SyntaxError: use of super property/member accesses only valid within methods or eval code within methods
    91. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    92. ReferenceError: deprecated caller or arguments usage
    93. TypeError: 'x' is not iterable
    94. TypeError: "x" is (not) "y"
    95. TypeError: "x" is not a constructor
    96. TypeError: "x" is not a function
    97. TypeError: "x" is not a non-null object
    98. TypeError: "x" is read-only
    99. TypeError: already executing generator
    100. TypeError: BigInt value can't be serialized in JSON
    101. TypeError: calling a builtin X constructor without new is forbidden
    102. TypeError: can't access/set private field or method: object is not the right class
    103. TypeError: can't assign to property "x" on "y": not an object
    104. TypeError: can't convert BigInt to number
    105. TypeError: can't convert x to BigInt
    106. TypeError: can't define property "x": "obj" is not extensible
    107. TypeError: can't delete non-configurable array element
    108. TypeError: can't redefine non-configurable property "x"
    109. TypeError: can't set prototype of this object
    110. TypeError: can't set prototype: it would cause a prototype chain cycle
    111. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    112. TypeError: class constructors must be invoked with 'new'
    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
    115. TypeError: getting private setter-only property
    116. TypeError: Initializing an object twice is an error with private fields/methods
    117. TypeError: invalid 'instanceof' operand 'x'
    118. TypeError: invalid Array.prototype.sort argument
    119. TypeError: invalid assignment to const "x"
    120. TypeError: Iterator/AsyncIterator constructor can't be used directly
    121. TypeError: matchAll/replaceAll must be called with a global RegExp
    122. TypeError: More arguments needed
    123. TypeError: "x" has no properties
    124. TypeError: property "x" is non-configurable and can't be deleted
    125. TypeError: Reduce of empty array with no initial value
    126. TypeError: setting getter-only property "x"
    127. TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol
    128. TypeError: X.prototype.y called on incompatible type
    129. URIError: malformed URI sequence
    130. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    131. Warning: unreachable code after return statement
    1. JavaScript
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