In javascript, filter and map an array in one function?

Question

In Javascript, Array.filter() takes an array and filters it down based on a certain criteria.

const a = [1,2,3,4,5].filter(el => el > 3);
console.log(a);

Result: [4,5]

Array.map() takes an array and returns a new array of equal length, usually mutating the original's elements in the process.

const a = [1,2,3,4].map(el => el + 10);
console.log(a);

Result: [11,12,13,14,15]

My question is, besides combining the two functions like this:

let a = [1,2,3,4,5].filter(el => el > 3).map(el => el + 10);
console.log(a);

is there an efficient way to filter and mutating an array, that doesn't involve multiple lines of code like most Array.forEach, for, and for..in routines? I know that Array.filter().map() is pretty efficient, I'm just wondering if it can be further streamlined.

Unless there is a measurable reason to seek an optimization filter().map() is good — it's very readable and easy to understand. — Mark
– Mark, Commented Dec 7, 2019 at 3:21

Mr. Polywhirl · Accepted Answer · 2019-12-07 03:25:33Z

2

Use Array.prototype.reduce to take care of filtering and mapping all at once.

let a = [1,2,3,4,5].reduce((arr, el) => el > 3 ? arr.concat(el + 10) : arr, []);
console.log(a);

You could also make your own mapIf polyfill function.

// Reduce Only
if (Array.prototype.mapIf === undefined) {
  Array.prototype.mapIf = function(predicateFn, applyFn) {
    return this.reduce((ref, el) => predicateFn(el) ? ref.concat(applyFn(el)) : ref, []);
  };
}

// Filter + Map
if (Array.prototype.mapIf === undefined) {
  Array.prototype.mapIf = function(predicateFn, applyFn) {
    return this.filter(predicateFn).map(applyFn);
  };
}


let a = [1,2,3,4,5].mapIf(el => el > 3, el => el + 10);
console.log(a);

edited Dec 7, 2019 at 3:25

answered Dec 7, 2019 at 3:19

Mr. Polywhirl

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