How do I multiply each member of an array by a scalar in javascript?

Question

For example, how do I achieve the following without iterating over the array?

var a = [1, 2, 3] * 5;  // a should equal [5, 10, 15]

Answer 1

You can try this:

function scalarMultiply(arr, multiplier) {
   for (var i = 0; i < arr.length; i++)
   {
      arr[i] *= multiplier;
   }
   return arr;
}

USAGE

var a = scalarMultiply([1, 2, 3], 5);

Answer 2

Array.map() is available to IE users as of IE9, so if you don't care about compatibility at all you can use this:

var a = [1, 2, 3].map(function(x) { return x * 5; });

For JavaScript 1.8, this is as short as you can go:

var a = [1, 2, 3].map(function(x) x * 5);

If you need maximal browser compatibility, you'll have to put up with a loop.

Either way you'll be iterating over the array; Array.map() just makes it less obvious you're doing so.

Answer 3

In ECMAScript 6, you can use arrow functions:

var a = [1, 2, 3];
var b = a.map(x => x * 5); // <-------

console.log(b);   // [5, 10, 15]

---

Arrow functions are syntactic sugar for an inline function with lexical this binding:

// ES6
let array2 = array1.map(x => x * 5);

// ES5
var array2 = array1.map((function (x) { return x * 5; }).bind(this));

Therefore, if you need to support Internet Explorer or other old browsers (Edge understands ES6) you can use babeljs or TypeScript in your project to cross-compile your code to ES5 syntax.