JS: Spread Operator (triple dots)

(new in ECMAScript 2015)

...iterable

insert the values of a Iterable Object into a array, or as arguments to a function.

• [1,...[2,3],4] becomes [1,2,3,4].
• f(1,...[2,3],4) becomes f(1,2,3,4).

/* insert an array's elements into another. */
console.log([2, ...[3, 4], 6]);
// [ 2, 3, 4, 6 ]

/* feed array to a function as multiple args */
console.log(((x, y) => (x + y))(...[3, 4]));
// 7

/* turn string into array of chars */
console.log([..."a🦋c"]);
// [ "a", "🦋", "c" ]

/* insert map entries into array */
console.log([9, ...(new Map([[3, 2376], [4, 7240]]))]);
// [ 9, [ 3, 2376 ], [ 4, 7240 ] ]

...object

(new in ECMAScript 2018)

insert Enumerable properties of object into a Object Literal Expression.

e.g. { ...{ a:1, b:2}, c:3} becomes { a:1, b:2, c:3}.

Later key overrides previous.

/* Merge Objects */
console.log({ b: 3, ...{ a: 1, b: 2 }, b: 4 });
// { b: 4, a: 1 }

// later key overrides previous