Modern features of JavaScript

The spread operator, which is denoted by three dots … , is a very versatile and powerful feature of modern JavaScript.

It has four major uses:

1. Function application
2. Working with arrays*
3. Working with objects

* array spreads are just a special case of spreads with Iterables (more soon).

Function application

Function application simply means calling functions but JavaScript functions come with an apply() method that enables you to call that function while passing an array for arguments.

f = (a, b, c) => a + b + c;

args = [ 1, 2, 3 ]

f.apply(null, args);

Result

> 6

You can use the spread operator as an alternative to apply().

f(...args);

You can also mix and match your arguments

f(10, ...[20, 30])

f(...[10, 20], 30)

f(10, ...[20], 30)

Result

> 60

Working with Arrays

For a given array:

arr1 = [ 1, 2, 3 ];

you can use the spread operator to create a shallow copy of the array

arr2 = [ ...arr2 ];

Result

> arr2 = [ 1, 2, 3 ];

You can also use the spread operator to prepend an element to an array

arr1 = [ 1, 2, 3 ];

arr2 = [ 0, ...arr1 ];

Result

> arr2 = [ 0, 1, 2, 3 ]

Or you can use the spread operator to append an element to an array

arr1 = [ 1, 2, 3 ];

arr2 = [ ...arr1, 4 ];

Result

> arr2 = [ 1, 2, 3, 4 ]

you can also use the spread operator to concatenate multiple arrays

arr1 = [ 1, 2, 3 ];
arr2 = [ 4, 5, 6 ];
arr3 = [ 7, 8, 9 ];

arr4 = [ ...arr1, ...arr2, ...arr3 ];

Result

> arr4 = [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

The spread operator can be used in any location of the receiving object and be mixed with normal key assignments

arr1 = [ 1, 2, 3 ];
arr2 = [ 4, 5, 6 ];

arr3 = [ 100, ...arr1, 200, ...arr2, 300 ];

Result

> arr3 = [ 100, 1, 2, 3, 200, 4, 5, 6, 300 ]

Working with Objects

The spread operator for objects works in a very similar way to the spread operator for arrays/iterables but its behaviour is subtly different. It is useful for spreading the key-value pairs of an object into another object

For a given object:

obj1 = { a: 1, b: 2, c: 3 };

you can create a shallow copy using the spread operator

obj2 = { ...obj1 };

Result

> obj2 = { a:1, b:2, c:3 }

you can also use the spread operator to merge multiple objects together

obj1 = { a: 1, b: 2, c: 3 };
obj2 = { d: 4, e: 5, f: 6 };
obj3 = { g: 7, h: 8, i: 9 };

obj4 = { ...obj1, ...ob2, ...obj3 };

Result

> obj4 = { a:1, b:2, c:3, d:4, e:5, f:6, g: 7, h: 8, i: 9 }

you can use the spread operator in any location of the receiving object and mix it with normal key-value assignments

obj1 = { a: 1, b: 2, c: 3 };
obj2 = { d: 4, e: 5, f: 6 };
a = 100
b = 200

obj3 = { a, ...obj1, b, ...ob2, c: 300 };

Result

> obj3 = { a: 1, b: 200, c: 300, d: 4, e: 5, f: 6 }

NOTE: If keys get repeated the value gets overridden with the last assigned value.

NOTE: Key order is preserved in JavaScript based on insertion order.

obj1 = { a: 1, b: 2, c: 3 };

obj2 = { c: 10, ...obj1 }

Result

> obj2 = { c: 3, a: 1, b: 2 }

Object.keys(obj2);

> [ 'c', 'a', 'b' ]

Object Spreads with other object types

Because the spread operator for objects treats its argument as a simple object. This means that it treats arrays and other iterable types as objects which have their index as a key.

Given the array

arr1 = [ 10, 20, 30 ]

The object spread operator will treat the array as simple object

obj1 = { ...arr1 };

Result

> obj1 = { 0: 10, 1: 20, 2: 30 }

Given the string

str1 = 'Hello';

The object spread operator will treat the string as simple object

obj1 = { ...str1 };

Result

> obj1 = { 0: 'H', 1: 'e', 2: 'l', 3: 'l', 4: 'o' }