cheat-sheet-arrays.md

Cheat sheet: Arrays

JavaScript Arrays are a very flexible data structure and used as lists, stacks, queues, tuples (e.g. pairs), etc. Some

Using Arrays

Creating Arrays, reading and writing elements:

const arr = ['a', 'b', 'c']; // Array literal
assert.deepEqual(
  arr,
  [
    'a',
    'b',
    'c', // trailing commas are ignored
  ]
);
assert.equal(
  arr.length, 3 // number of elements
);
assert.equal(
  arr[0], 'a' // read (negative indices don’t work)
);
assert.equal(
  arr.at(-1), 'c' // read (negative indices work)
);
arr[0] = 'x'; // write
assert.deepEqual(
  arr, ['x', 'b', 'c']
);

The lengths of Arrays, adding elements:

const arr = ['a'];
assert.equal(
  arr.length, 1 // number of elements
);
arr.push('b'); // add an element (preferred)
assert.deepEqual(
  arr, ['a', 'b']
);
arr[arr.length] = 'c'; // add an element (alternative)
assert.deepEqual(
  arr, ['a', 'b', 'c']
);
arr.length = 1; // remove elements
assert.deepEqual(
  arr, ['a']
);

Concatenating Arrays via spreading (...):

const arr1 = ['a', 'b'];
const arr2 = ['c'];
const arr3 = ['d', 'e', 'f'];
assert.deepEqual(
  [...arr1, ...arr2, ...arr3, 'g'],
  ['a', 'b', 'c', 'd', 'e', 'f', 'g']
);

Clearing Arrays (removing all elements):

// Affects everyone referring to the Array
const arr1 = ['a', 'b', 'c'];
arr1.length = 0;
assert.deepEqual(
  arr1, []
);

// Does not affect others referring to the Array
let arr2 = ['a', 'b', 'c'];
arr2 = [];
assert.deepEqual(
  arr2, []
);

Looping over elements:

const arr = ['a', 'b', 'c'];
for (const value of arr) {
  console.log(value);
}

// Output:
// 'a'
// 'b'
// 'c'

Looping over key-element pairs:

const arr = ['a', 'b', 'c'];
for (const [index, value] of arr.entries()) {
  console.log(index, value);
}

// Output:
// 0, 'a'
// 1, 'b'
// 2, 'c'

Creating and filling Arrays when we can’t use Array literals (e.g. because we don’t know their lengths in advance or they are too large):

const four = 4;

// Empty Array that we’ll fill later
assert.deepEqual(
  new Array(four),
  [ , , , ,] // four holes; last comma is ignored
);

// An Array filled with a primitive value
assert.deepEqual(
  new Array(four).fill(0),
  [0, 0, 0, 0]
);

// An Array filled with objects
// Why not .fill()? We’d get single object, shared multiple times.
assert.deepEqual(
  Array.from({length: four}, () => ({})),
  [{}, {}, {}, {}]
);

// A range of integers
assert.deepEqual(
  Array.from({length: four}, (_, i) => i),
  [0, 1, 2, 3]
);

Array methods

Deriving a new Array from an existing Array:

> ['■','●','▲'].slice(1, 3)
['●','▲']
> ['■','●','■'].filter(x => x==='■') 
['■','■']

> ['▲','●'].map(x => x+x)
['▲▲','●●']
> ['▲','●'].flatMap(x => [x,x])
['▲','▲','●','●']

Removing an Array element at a given index:

const arr = ['■','●','▲'];

assert.deepEqual(
  arr.filter((_, index) => index !== 1),
  ['■','▲']
);
assert.deepEqual(
  arr, ['■','●','▲'] // .filter() is non-destructive
);

arr.splice(1, 1); // start at 1, delete 1 element
assert.deepEqual(
  arr, ['■','▲'] // .splice() is destructive
);

Computing a summary of an Array:

> ['■','●','▲'].some(x => x==='●')
true
> ['■','●','▲'].every(x => x==='●')
false

> ['■','●','▲'].join('-')
'■-●-▲'

> ['■','●'].reduce((result,x) => result+x, '▲')
'▲■●'
> ['■','●'].reduceRight((result,x) => result+x, '▲')
'▲●■'

Changing all of an Array (the input Array is modified and returned):

> ['■','●','▲'].fill('●')
['●','●','●']
> ['■','●','▲'].reverse()
['▲','●','■']
> ['■','●','■'].sort()
['■','■','●']

Finding Array elements:

> ['■','●','■'].includes('■')
true
> ['■','●','■'].indexOf('■')
0
> ['■','●','■'].lastIndexOf('■')
2
> ['■','●','■'].find(x => x==='■')
'■'
> ['■','●','■'].findIndex(x => x==='■')
0

Listing elements (Array.from() is needed because the methods return iterables, not Arrays):

> Array.from(['■','●','▲'].keys())
[0,1,2]
> Array.from(['■','●','▲'].entries())
[[0,'■'],[1,'●'],[2,'▲']]

Adding or removing an element at either end of an Array:

const arr1 = ['■','●'];
arr1.push('▲');
assert.deepEqual(
  arr1, ['■','●','▲']
);

const arr2 = ['■','●','▲'];
arr2.pop();
assert.deepEqual(
  arr2, ['■','●']
);

const arr3 = ['■','●'];
arr3.unshift('▲');
assert.deepEqual(
  arr3, ['▲','■','●']
);

const arr4 = ['▲','■','●'];
arr4.shift();
assert.deepEqual(
  arr4, ['■','●']
);

array methods all in one

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array

1. flat

// flat
const arr = [0, 1, 2, [[[3, 4]]]];

console.log(arr.flat());
console.log(arr.flat(2));
console.log(arr.flat(3));
console.log(arr.flat(Infinity));

2. copyWithin

// copyWithin
const arr = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N'];

// copyWithin(target, start, end)
console.log([...arr].copyWithin(0, 1, 3));

// copyWithin(target, start)
console.log([...arr].copyWithin(0, 2));

//  copyWithin(target)
//  ⚠️ under some condition, it equal to `copyWithin(target, 0, array's index)`
console.log([...arr].copyWithin(3));
console.log([...arr].copyWithin(3, 0, arr.length));