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array-cheat-sheet.md

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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

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