String

Return Type: String

MethodDescriptionExampleResult
charAt(index)Character at position (empty string if out of range).'Hello'.charAt(1)'e'
concat(...strings)Concatenate strings.'Hello'.concat(' ', 'World')'Hello World'
slice(start, end?)Extract substring by indices (end not included).'Hello'.slice(1, 4)'ell'
substring(start, end?)Extract substring; swaps args if start > end.'Hello'.substring(1, 4)'ell'
toUpperCase()Uppercase copy.'hi'.toUpperCase()'HI'
toLowerCase()Lowercase copy.'HI'.toLowerCase()'hi'
trim()Remove leading/trailing whitespace.' hi '.trim()'hi'
trimStart(), trimEnd()Remove whitespace at start or end.' hi'.trimStart()'hi'
padStart(targetLength, padString?)Pad at start to reach length.'5'.padStart(3, '0')'005'
padEnd(targetLength, padString?)Pad at end to reach length.'5'.padEnd(3, '0')'500'
repeat(count)Repeat string count times.'ab'.repeat(3)'ababab'
replace(searchValue, replaceValue)Replace first match (string or regex).'hi hi'.replace('hi','yo')'yo hi'
replaceAll(searchValue, replaceValue)Replace all matches.'hi hi'.replaceAll('hi','yo')'yo yo'
normalize(form?)Unicode normalization (NFC, NFD, NFKC,NFKD).'é'.normalize('NFD')'é'
toString(), valueOf()Primitive string value.new String('x').toString()'x'

Return Type: Number

MethodDescriptionExampleResult
parseInt(string)Parses a string and returns an integer.parseInt('77')77
parseFloat(string)Parses a string and returns a floating point number.parseFloat('4.567')4.567
charCodeAt(index)UTF‑16 code unit at position.'ABC'.charCodeAt(0)65
indexOf(searchValue, fromIndex?)First index of substring (or -1).'Hello'.indexOf('l')2
lastIndexOf(searchValue, fromIndex?)Last index of substring (or -1).'Hello'.lastIndexOf('l')3
localeCompare(other, locales?, options?)Compares strings using locale rules.'a'.localeCompare('b')-1 (read notes)
search(regexp)Index of first regex match (or -1).'abc'.search(/b/)1

Return Type: Boolean

MethodDescriptionExampleResult
includes(substring, fromIndex?)Substring presence.'Hello'.includes('ell')true
startsWith(prefix, position?)Check starting substring.'Hello'.startsWith('He')true
endsWith(suffix, length?)Check ending substring.'Hello'.endsWith('lo')true

Return Type: Array

MethodDescriptionExampleResult
split(separator, limit?)Split into array of substrings.'a,b,c'.split(',')['a','b','c']

Return Type: Iterator

MethodDescriptionExampleResult
matchAll(regexp)Returns an iterator of all regex matches.Array.from('test'.matchAll(/t/g))[['t'],['t']] (after Array.from)

Return Type: Array ornull

MethodDescriptionExampleResult
match(regexp)First regex match result array (or null if none).'abc'.match(/b/)['b', index:1, input:'abc', groups:undefined] (or null)

Notes

  • Most string methods are case-sensitive by default. To do case-insensitive checks, use regex with thei flag or convert both strings to a common case withtoLowerCase()/toUpperCase().
  • String.prototype.at(index) can return a string or undefined (if out of range), so it is omitted from single-type tables.
  • substr() is deprecated; prefer slice() or substring().
  • localeCompare returns different values depending on whether the reference value is before/after/equal to the compare value
    • -1: if the reference value is before the compared value (i.e. 'a'.localeCompare('b'))
    • 0: if the reference is equal to the compared (i.e. 'b'.localeCompare('b'))
    • 1: if the reference is after the compared (i.e. 'c'.localeCompare('b'))

Array

Return Type: Boolean

MethodDescriptionExampleResultCallback Arguments
every(callbackFn, thisArg?)Checks if all elements pass the test.[1, 2, 3].every(x => x > 0)true
  • element — current element
  • index — index of element
  • array — original array
some(callbackFn, thisArg?)Checks if at least one element passes the test.[1, 2, 3].some(x => x > 2)true
  • element — current element
  • index — index of element
  • array — original array
includes(valueToFind, fromIndex?)Checks if the array contains a value.[1, 2, 3].includes(2)true-

Return Type: Number

MethodDescriptionExampleResultCallback Arguments
indexOf(searchElement, fromIndex?)Returns the first index of the element, or -1 if not found.['a', 'b', 'c'].indexOf('b')1-
lastIndexOf(searchElement, fromIndex?)Returns the last index of the element, or -1 if not found.['a', 'b', 'a'].lastIndexOf('a')2-
findIndex(callbackFn, thisArg?)Returns the index of the first element that satisfies the test, or -1.[4, 6, 8].findIndex(x => x > 5)1
  • element — current element
  • index — index of element
  • array — original array
reduce(callbackFn, initialValue)Reduce an array to a single value by applying a function to each element, accumulating the result.[1,2].reduce((acc, curr) => acc + curr, 0)3
  • accumulator — accumulated value
  • currentValue — current element
  • currentIndex — index of element
  • array — original array

Return Type: String

MethodDescriptionExampleResult
join(separator?)Joins all elements into a string.['a', 'b', 'c'].join('-')'a-b-c'
toString()Converts the array to a comma-separated string.[1, 2, 3].toString()'1,2,3'
toLocaleString()Converts array to a locale-sensitive string.[1234.56].toLocaleString()'1,234.56' (US locale)

Return Type: Array

MethodDescriptionExampleResultCallback Arguments
filter(callbackFn, thisArg?)Creates a new array with elements that pass the test.[1, 2, 3, 4].filter(x => x > 2)[3, 4]
  • element — current element
  • index — index of element
  • array — original array
map(callbackFn, thisArg?)Creates a new array with results of calling a function on every element.[1, 2, 3].map(x => x * 2)[2, 4, 6]
  • element — current element
  • index — index of element
  • array — original array
slice(start?, end?)Returns a shallow copy of a portion of the array.['a', 'b', 'c'].slice(1)['b', 'c']-

Return Type: Element

MethodDescriptionExampleResultCallback Arguments
find(callbackFn, thisArg?)Returns the first element that satisfies the test, or undefined.[5, 12, 8].find(x => x > 10)12
  • element — current element
  • index — index of element
  • array — original array
pop()Removes the last element and returns it.let arr = [1, 2, 3]; arr.pop()3-
shift()Removes the first element and returns it.let arr = [1, 2, 3]; arr.shift()1-

Return Type: Number (Length)

MethodDescriptionExampleResult
push(...items)Adds one or more elements to the end, returns new length.let arr = [1]; arr.push(2, 3)3
unshift(...items)Adds one or more elements to the start, returns new length.let arr = [1]; arr.unshift(0)2

Return Type: Undefined (no return)

MethodDescriptionExampleResultCallback Arguments
forEach(callbackFn)Act on each element using the provided function.[1, 2, 3].forEach((element) => console.log(element));1 2 3
  • element — current element
  • index — index of element
  • array — original array

Notes

  • A shallow copy creates a new object, but it doesn't duplicate nested objects; it copies references to them
    • i.e. changes to the nested objects in the copy also affect the original
  • Arrays in JavaScript are zero-indexed.
  • Other methods (map, filter, slice, concat) return a new array without changing the original.

Set

Return Type: Boolean

MethodDescriptionExampleResult
has(value)Checks if the Set contains a value.new Set([1, 2, 3]).has(2)true
delete(value)Removes a value from the Set. Returns true if found and removed, otherwisefalse.let s = new Set([1]); s.delete(1)true

Return Type: Number

MethodDescriptionExampleResult
sizeReturns the number of elements in the Set.new Set([1, 2, 3]).size3

Return Type: Set (itself) or undefined

MethodDescriptionExampleResultCallback Arguments
add(value)Adds a new value to the Set and returns the Set object.let s = new Set(); s.add(1)Set 1-
clear()Removes all elements from the Set and returns undefined.let s = new Set([1, 2]); s.clear()undefined-
forEach(callbackFn)Act on each element using the provided function.new Set([1, 2, 3]).forEach((element) => console.log(element));1 2 3
  • value — current element
  • key — same as value (for compatibility with Map)
  • set — original Set

Return Type: Iterator

MethodDescriptionExampleResult
values()Returns a new Iterator object containing all values.[...new Set([1, 2]).values()][1, 2]
keys()Alias for values() in Set.[...new Set(['a', 'b']).keys()]['a', 'b']
entries()Returns a new Iterator of [value, value] pairs.[...new Set(['x', 'y']).entries()][['x', 'x'], ['y', 'y']]

Notes

  • Set objects store UNIQUE values of any type, whether primitive values or object references.
  • The insertion order is preserved when iterating over a Set.
  • Set.prototype.size is a property, not a method.

Map

Return Type: Boolean

MethodDescriptionExampleResult
has(key)Returns true if a Map contains the specified key.new Map([[1, 'a']]).has(1)true
delete(key)Removes a Map element by key and returns true if removed.let m = new Map([[1,'a']]); m.delete(1)true

Return Type: Number

MethodDescriptionExampleResult
sizeReturns the number of key/value pairs in the Map.new Map([[1,'a'], [2,'b']]).size2

Return Type: Map (itself) or undefined

MethodDescriptionExampleResult
set(key, value)Adds or updates an element with a specified key and value.let m = new Map(); m.set(1,'a')Map {1 => 'a' &125;
clear()Removes all Map elements.let m = new Map([[1,'a']]); m.clear()undefined

Return Type: Iterator

MethodDescriptionExampleResultCallback Arguments
keys()Returns a new Iterator object containing the keys of the Map.myMap = new Map([[1,'a'],[2,'b']]); myMap.keys();[1,2]-
values()Returns a new Iterator object containing the values of the Map.myMap = new Map([[1,'a'],[2,'b']]); myMap.values();['a','b']-
entries()Returns a new Iterator object containing [key, value] pairs for each element.myMap = new Map([[1,'a'],[2,'b']]); myMap.entries();[[1,'a'],[2,'b']]-
forEach(callback, thisArg?)Calls a function for each key/value pair in the Map.myMap = new Map([[1,'a']]); myMap.forEach((v,k) => console.log(k,v))logs: 1 'a'
  • value — current value
  • key — current key
  • map — original Map

Notes

  • forEach(callback(value, key, map)): the key and value positions are flipped
  • Map keys can be of any value type, including objects and functions.
  • Insertion order of keys is preserved during iteration.
  • Use forEach or the iterator methods (keys, values,entries) to traverse a Map.
  • size is a property, not a method.

Object

Return Type: Boolean

MethodDescriptionExampleResult
hasOwnProperty(prop)Checks if the object has the specified property as its own property.({ a: 1 }).hasOwnProperty('a')true
propertyIsEnumerable(prop)Checks if a property is enumerable.({ a: 1 }).propertyIsEnumerable('a')true

Return Type: Array

MethodDescriptionExampleResult
Object.keys(obj)Returns an array of the object's own enumerable property names.Object.keys({ a: 1, b: 2 })['a', 'b']
Object.values(obj)Returns an array of the object's own enumerable property values.Object.values({ a: 1, b: 2 })[1, 2]
Object.entries(obj)Returns an array of the object's own enumerable [key, value] pairs.Object.entries({ a: 1 })[['a', 1]]

Return Type: Object

MethodDescriptionExampleResult
Object.assign(target, ...sources)Copies properties from source objects to a target object.Object.assign({}, { a: 1 }, { b: 2 }){ a: 1, b: 2 }
Object.create(proto, propertiesObject?)Creates a new object with the specified prototype object and properties.Object.create({ a: 1 })Object with prototype { a: 1 }
Object.fromEntries(iterable)Returns a new object from an iterable of [key, value] pairs.Object.fromEntries([['a', 1]]){ a: 1 }

Return Type: String or Primitive

MethodDescriptionExampleResult
toString()Returns a string representation of the object.({ a: 1 }).toString()'[object Object]'
valueOf()Returns the primitive value of the specified object.({ a: 1 }).valueOf(){ a: 1 }

Notes

  • Some Object methods are static (e.g., Object.keys), while others are instance methods (e.g., toString).

JSON

Return Type: String

MethodDescriptionExampleResult
JSON.stringify(value, replacer?, space?)Converts a JavaScript value to a JSON string.JSON.stringify({ a: 1 })'{'a':1}'

Return Type: Object

MethodDescriptionExampleResult
JSON.parse(text, reviver?)Parses a JSON string and returns the corresponding JavaScript value.JSON.parse('{'a':1}'){ a: 1 }

Notes

  • All JSON methods are static and are called on the JSON object.
  • JSON.stringify can take a replacer function/array to filter properties, and aspace argument to format the output.
  • JSON.parse can take a reviver function to transform values during parsing.
  • JSON only supports strings, numbers, booleans, null, arrays, and objects (no functions orundefined).

Math

Return Type: Number

FunctionDescriptionExampleResult
Math.abs(x)Returns the absolute value of a number.Math.abs(-5)5
Math.ceil(x)Returns the smallest integer greater than or equal to a number.Math.ceil(4.2)5
Math.floor(x)Returns the largest integer less than or equal to a number.Math.floor(4.8)4
Math.round(x)Returns the value of a number rounded to the nearest integer.Math.round(4.5)5
Math.max(...values)Returns the largest of zero or more numbers.Math.max(1, 3, 2)3
Math.min(...values)Returns the smallest of zero or more numbers.Math.min(1, 3, 2)1
Math.random()Returns a pseudo-random number between 0 (inclusive) and 1 (exclusive).Math.random()0.123... (varies)
Math.sqrt(x)Returns the positive square root of a number.Math.sqrt(9)3
Math.pow(base, exponent)Returns the base raised to the exponent power.Math.pow(2, 3)8
Math.PIReturns piMath.PI3.14...

Return Type: Boolean

MethodDescriptionExampleResult
Number.isFinite(value)Determines whether the passed value is a finite number.Number.isFinite(10)true
Number.isNaN(value)Determines whether the passed value is NaN.Number.isNaN(NaN)true
Number.isInteger(value)Determines whether the passed value is an integer.Number.isInteger(4)true

Notes

  • All Math methods are static and must be called on the Math object (e.g.,Math.sqrt(4)).
  • Math.random() does not produce cryptographically secure numbers. Usecrypto.getRandomValues() for secure random numbers.
  • Some numeric checks like isNaN() exist globally, but Number.isNaN() is more reliable because it does not coerce values.