Array

Question 1

Array.from()

Answer 1

create an array from an array-like

params: arrayLike[, mapFn[, thisArg]]

arrayLike
An array-like or iterable object to convert to an array.
mapFn
Optional. Map function to call on every element of the array.
thisArg
Optional. Value to use as this when executing mapFn.

Question 2

Array.isArray()

Answer 2

Determine if the passed in object is an array

params: obj

Note: necessary because an array from a different window might have an augmented Array.prototype, in which case instanceof might not work across the different windows.

Question 3

Array.observe()

Answer 3

Asynchronously observe changes

params: arr, callback

arr
The array to observe
callback
The callback. Takes an array of change objects. Change objects have [propertyName, changedArray, typeOfChange, oldValue (for update and delete types), index + removed + addedCount (for splice type)]

Question 4

Array.of()

Answer 4

Creates a new array from a list of args

params: element0[, …elementN]

Question 5

Array.prototype.concat()

Answer 5

Returns combined arrays

params: arr1[, …arrN]

Question 6

Array.prototype.copyWithin()

Answer 6

(Mutable) Copies part of an array to another part

params: target, start[, end]

target
    the index to start inserting
start
    the start of the copy range
end
    the end of the copy range, exclusive

Question 7

Array.prototype.entries()

Answer 7

Returns a new Array Iterator. Each iteration returns an array [k, v]

Question 8

Array.prototype.every()

Answer 8

Tests whether all elements pass a test

params: callback[, thisArg]

callback
args are [currentValue, index, arr]. return T/F
thisArg
the this binding for the callback

Question 9

Array.prototype.fill()

Answer 9

fill an array with a value

params: value[, start[, end]]

value
    value to use
start
    start index
end
    end index, exclusive

Question 10

Array.prototype.filter()

Answer 10

Return a new array with only elements that pass the test

params: callback[, thisArg]

callback
args [elem, index, arr]. returns T/F
thisArg
the this binding for the callback

Question 11

Array.prototype.find()

Answer 11

Returns a value from the array if an element satisfies the test

params: callback[, thisArg]

callback
Test (element, index, array)
thisArg
this binding

Question 12

Array.prototype.findIndex()

Answer 12

Returns the first index of an element that satisfies the test

params: callback[, thisArg]

callback
Test (element, index, array)
thisArg
this binding

Question 13

Array.prototype.forEach()

Answer 13

Executes a callback on each element of an array

params: callback[, thisArg]

callback
function (currentValue, index, arr)
thisArg
this binding

Question 14

Array.prototype.includes()

Answer 14

Returns true if the array includes a value

params: searchElement[, fromIndex]

Question 15

Array.prototype.indexOf()

Answer 15

Returns first index of an element

params: searchElement[, fromIndex=0]

Question 16

Array.prototype.join()

Answer 16

joins all elements into a string

params: [separator = ‘,’]

Note: separator is converted to string

Question 17

Array.prototype.keys()

Answer 17

Returns an array iterator with keys for each index

Note: does not ignore holes like Object.keys() does

Question 18

Array.prototype.lastIndexOf()

Answer 18

Returns last index of fiven element

params: searchElement[, fromIndex = arr.length-1]

Question 19

Array.prototype.map()

Answer 19

Creates a new array with results of calling function on each element in array

params: callback[, thisArg]

callback
(currentValue, index, array)

Question 20

Array.prototype.pop()

Answer 20

Removes & returns last element

Note: intentionally generic, so you can call/apply it on objects resembling arrays

Question 21

Array.prototype.push()

Answer 21

Adds one or more elements. Returns new length

params: element1, …, elementN

Note: intentionally generic, so you can call/apply on objects resembling arrays

Question 22

Array.prototype.reduce()

Answer 22

Reduces an array to a single value

params: callback[, initialValue]

callback
(prevVal, currentVal, index, arr)
initialValue
first prevVal

Question 23

Array.prototype.reduceRight()

Answer 23

reduce from the right

Question 24

Array.prototype.reverse()

Answer 24

Reverse an array in place

Question 25

Array.prototype.shift()

Answer 25

Remove and return first element

Note: intentionally generic

Question 26

Array.prototype.slice()

Answer 26

Returns a shallow copy of part of the array

params: begin[, end]

begin
if negative, offset is from end. (-2) returns last two
end
exclusive. if negative, offset is from end

Question 27

Array.prototype.some()

Answer 27

Test whether at least one element passes a test

params: callback[, thisArg]

callback
(value, i, arr)
thisArg
this binding

Question 28

Array.prototype.sort()

Answer 28

Sorts an array in place

params: [compareFunction]

compareFunction
(a,b)

Note: numbers are converted to strings

Question 29

Array.prototype.splice()

Answer 29

Changes an array by adding/removing elements. Returns removed elements

params: start[, deleteCount[, item1[, itemN]]]

start
    if negative, will begin that many elements from end
deleteCount
    number of elements to delete
itemN
    elements to add

Question 30

Array.prototype.toLocaleString()

Answer 30

Returns a string representing elements in the array in locale-specific format

Question 31

Array.prototype.toString

Answer 31

outputs comma separated elements

Question 32

Array.prototype.unshift()

Answer 32

Adds elements to the beginning of a string

params: item1[, itemN]

Note: intentionally generic

Question 33

Array.prototype.values()

Answer 33

Returns an iterator with the values

Question 34

Array.unobserve

Answer 34

Remove observers set by observe

params: arr, callback

arr
the array to stop observing
callback
The reference to the observer that should not be called anymore