Spread Operator의 주요 목적은 Array이나 Object의 요소를 퍼뜨리는 것이다. 이것은 예시로 가장 잘 설명된다.

일반적인 사용 사례는 함수 인수에 배열을 퍼뜨리는 것이다. 이전에는 Function.prototype.apply 사용해야 합니다:

function foo(x, y, z) { }
var args = [0, 1, 2];
foo.apply(null, args);

이제는 아래와 같이 인수앞에 …을 붙여서 사용하면 된다.

function foo(x, y, z) { }
var args = [0, 1, 2];
foo(...args);

여기서 우리는 배열을 인수로 넘겨주었다.

우리는 destructuring의 한가지 사용법을 보았다.

var [x, y, ...remaining] = [1, 2, 3, 4];
console.log(x, y, remaining); // 1,2,[3,4]

The motivation here is to simply make it easy for you to capture the remaining elements of an array when destructuring. Array Assignment The spread operator allows you to easily place an expanded version of an array into another array. This is demonstrated in the example below:

var list = [1, 2];
list = [...list, 3, 4];
console.log(list); // [1,2,3,4]

You can put the expanded array in at any position, and get the effect you'd expect:

var list = [1, 2];
list = [0, ...list, 4];
console.log(list); // [0,1,2,4]

You can also spread an object into another object. A common use case is to simply add a property to an object without mutating the original:

const point2D = {x: 1, y: 2};
/** Create a new object by using all the point2D props along with z */
const point3D = {...point2D, z: 3};

For objects, the order of where you put the spread matters. This works something like Object.assign, and does what you'd expect: what comes first is 'overridden' by what comes later:

const point2D = {x: 1, y: 2};
const anotherPoint3D = {x: 5, z: 4, ...point2D};
console.log(anotherPoint3D); // {x: 1, y: 2, z: 4}
const yetAnotherPoint3D = {...point2D, x: 5, z: 4}
console.log(yetAnotherPoint3D); // {x: 5, y: 2, z: 4}

Another common use case is a simple shallow extend:

const foo = {a: 1, b: 2, c: 0};
const bar = {c: 1, d: 2};
/** Merge foo and bar */
const fooBar = {...foo, ...bar};
// fooBar is now {a: 1, b: 2, c: 1, d: 2}

apply is something that you often use in JavaScript, so it's good to have a better syntax where you don't have that ugly null for the this argument. Also having a dedicated syntax for moving arrays out of (destructuring) or into (assignment) other arrays provides a neat syntax for when you are doing array processing on partial arrays.