Why no Array.prototype.flatMap in javascript?

Question

flatMap is incredibly useful on collections, but javascript does not provide one while having Array.prototype.map. Why?

Is there any way to emulate flatMap in javascript in both easy and efficient way w/o defining flatMap manually?

Answer 1

Update: Array.prototype.flatMap made it into ES2019

It is widely supported in many environments. See if it works in your browser using this snippet below -

const data =   [ 1, 2, 3, 4 ]    console.log(data.flatMap(x => Array(x).fill(x))) // [ 1, 2, 2, 3, 3, 3, 4, 4, 4, 4 ]

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"Why no Array.prototype.flatMap in javascript?"

Because programming isn't magic and every language doesn't have features/primitives that every other language has. What matters is JavaScript gives you the ability to define it on your own -

const concat = (x,y) =>   x.concat(y)  const flatMap = (f,xs) =>   xs.map(f).reduce(concat, [])  const xs = [1,2,3]  console.log(flatMap(x => [x-1, x, x+1], xs))

Or a rewrite that collapses the two loops into one -

const flatMap = (f, xs) =>   xs.reduce((r, x) => r.concat(f(x)), [])  const xs = [1,2,3]  console.log(flatMap(x => [x-1, x, x+1], xs))

If you want it to extend the Array.prototype, nothing is stopping you -

if (!Array.prototype.flatMap) {   function flatMap (f, ctx) {     return this.reduce       ( (r, x, i, a) =>           r.concat(f.call(ctx, x, i, a))       , []       )   }   Array.prototype.flatMap = flatMap }  const ranks =   [ 'J', 'Q', 'K', 'A' ]    const suits =   [ '♡', '♢', '♤', '♧' ]  const result =   ranks.flatMap(r =>     suits.flatMap(s =>       [[r, s]]     )   )  console.log(JSON.stringify(result)) // [ ['J','♡'], ['J','♢'], ['J','♤'], ['J','♧'] // , ['Q','♡'], ['Q','♢'], ['Q','♤'], ['Q','♧'] // , ['K','♡'], ['K','♢'], ['K','♤'], ['K','♧'] // , ['A','♡'], ['A','♢'], ['A','♤'], ['A','♧'] // ]