array-cheat-sheet.md

Cheat sheet: JavaScript Array methods

Deriving a new Array from an existing Array:

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

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 (spreading via ... is needed because the methods return iterables):

[...['■','●','▲'].keys()]    ⟼ [0,1,2]
[...['■','●','▲'].values()]  ⟼ ['■','●','▲']
[...['■','●','▲'].entries()] ⟼ [[0,'■'],[1,'●'],[2,'▲']]

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

arr=['■','●'];     arr.push('▲');    arr ⟼ ['■','●','▲']
arr=['■','●','▲']; arr.pop();        arr ⟼ ['■','●']

arr=['■','●'];     arr.unshift('▲'); arr ⟼ ['▲','■','●']
arr=['▲','■','●']; arr.shift();      arr ⟼ ['■','●']

Check out this quick reference for Array methods

My book “JavaScript for impatient programmers” contains a comprehensive quick reference:

• Lists all Array methods (a few methods were omitted in the cheat sheet)
• Brief explanations
• Code examples
• Method type signatures

Well done! This might be then most instructive and efficient cheat sheet I’ve ever seen.

cool! bit clearer with Colors/Emojis: https://array-methods.github.io/

Missing the gotcha arguments that are the index and source arguments for the callback (using parseInt as a map callback is the classic gotcha), and perhaps the Array constructor and from method on the constructor.

Also a really good sheet is this https://medium.com/@alvaro.saburido/set-theory-for-arrays-in-es6-eb2f20a61848

It shows things like Intersection, Difference, Symmetrical Difference and Union