Spread syntax with arrays
basically
expands the items of an array by prefixing it with ...
const middle = [1, 2, 3];
const arr = [0, middle, 4];
assert.deepEqual(arr, [0, 1, 2, 3, 4]);
an empty array expanded is no item
const arr = [0, [], 1];
assert.deepEqual(arr, [0, 1]);
is (in a way) the opposite to the rest syntax
both use ... to either expand all items and collect them
const [...rest] = [...[,1, 2, 3, 4, 5]];
assert.deepEqual(rest, [1, 2, 3, 4, 5]);
rest syntax must be last in an array, spread can be used in any place
const [a, b, ...rest] = [1, [2, 3], 4, 5];
assert.equal(a, 1);
assert.equal(b, 2);
assert.deepEqual(rest, [3, 4, 5]);
used as function parameter
prefix with ... to spread as function params
const magicNumbers = [1, 2];
const fn = (magicA, magicB) => {
assert.deepEqual(magicA, magicNumbers[0]);
assert.deepEqual(magicB, magicNumbers[1]);
};
fn(magicNumbers);
pass an array of numbers to Math.max()
const max = Math.max(...[23, 0, 42, 43]);
assert.equal(max, 42);
used as constructor parameter
just like in a function call (is not possible using `apply')
class X {
constructor(a, b, c) { return [a, b, c]; }
}
const args = [1];
assert.deepEqual(new X(...args), [1, 2, 3]);
Links
mdn
docs
Syntax docs on MDN.