JavaScript Arrays Array spread / rest
Example
Spread operator
6
With ES6, you can use spreads to separate individual elements into a comma-separated syntax:
let arr = [1, 2, 3, ...[4, 5, 6]]; // [1, 2, 3, 4, 5, 6]
// in ES < 6, the operations above are equivalent to
arr = [1, 2, 3];
arr.push(4, 5, 6);
The spread operator also acts upon strings, separating each individual character into a new string element. Therefore, using an array function for converting these into integers, the array created above is equivalent to the one below:
let arr = [1, 2, 3, ...[..."456"].map(x=>parseInt(x))]; // [1, 2, 3, 4, 5, 6]
Or, using a single string, this could be simplified to:
let arr = [..."123456"].map(x=>parseInt(x)); // [1, 2, 3, 4, 5, 6]
If the mapping is not performed then:
let arr = [..."123456"]; // ["1", "2", "3", "4", "5", "6"]
The spread operator can also be used to spread arguments into a function:
function myFunction(a, b, c) { }
let args = [0, 1, 2];
myFunction(...args);
// in ES < 6, this would be equivalent to:
myFunction.apply(null, args);
Rest operator
The rest operator does the opposite of the spread operator by coalescing several elements into a single one
[a, b, ...rest] = [1, 2, 3, 4, 5, 6]; // rest is assigned [3, 4, 5, 6]
Collect arguments of a function:
function myFunction(a, b, ...rest) { console.log(rest); }
myFunction(0, 1, 2, 3, 4, 5, 6); // rest is [2, 3, 4, 5, 6]
