You Don't Know JS: ES6 & Beyond

Chapter 6. API Additions

From conversions of values to mathematic calculations, ES6 adds many static properties and methods to various built-in natives and objects to help with common tasks. In addition, instances of some of the natives have new capabilities via various new prototype methods.

Note

Most of these features can be faithfully polyfilled. We will not dive into such details here, but check out “ES6 Shim” for standards-compliant shims/polyfills.

Array

One of the most commonly extended features in JS by various user libraries is the Array type. It should be no surprise that ES6 adds a number of helpers to Array, both static and prototype (instance).

Array.of(..) Static Function

There’s a well-known gotcha with the Array(..) constructor, which is that if there’s only one argument passed, and that argument is a number, instead of making an array of one element with that number value in it, it constructs an empty array with a length property equal to the number. This action produces the unfortunate and quirky “empty slots” behavior that’s reviled about JS arrays.

Array.of(..) replaces Array(..) as the preferred function-form constructor for arrays, because Array.of(..) does not have that special single-number-argument case. Consider:

var a = Array( 3 );
a.length;                       // 3
a[0];                           // undefined

var b = Array.of( 3 );
b.length;                       // 1
b[0];                           // 3

var c = Array.of( 1, 2, 3 );
c.length;                       // 3
c;                              // [1,2,3]

Under what circumstances would you want to use Array.of(..) instead ...