JavaScript Destructuring - ES6

Both arrays and objects now support destructuring.

Array destructuring gives a quicker and more fine-grained approach to interacting with elements in an array. See below:

const foo = ["one", "two", "three"];

const [a, b, c] = foo;

console.log(a + b + c); //=> "onetwothree"

const [x, ,z] = foo;

console.log(x + z); //=> "onethree"

const [, y] = foo;

console.log(y); //=> "two"

Object destructuring is similar to array destructuring but uses key names to do the destructuring.

const foo = {a: "1", b: "2", c: "3"};
const {a, c} = foo;

console.log(a + c); //=> "13"

// Assign a new variable name
const {b: baz} = foo;

console.log(baz); //=> "2"

Object destructuring also gives us some extra flexibility with nested objects and computed properties.

let foo = {a: "1", b: "2", c: "3"};

const computed = "d";

foo[computed] = "4";

// Destructure computed property
const {[computed]: bar} = foo;

console.log(bar); //=> "4"

const baz = {w: {x: "5", y: "6"}};

// Notice the overloaded ":" for nesting
const {w: {x: z}} = baz;

console.log(z); //=> "5"

Array and object destructuring can be used in conjuction with any nesting depth.

const foo = {a: [{b: "2"}, {c: ["3", "4"]}]};

const {a: [{b: x}, {c: [y, z] }] } = foo;

console.log(x); //=> "2"
console.log(y); //=> "3"
console.log(z); //=> "4"

Arrays also supports the rest operator .... This allows us to grab the remaining un-deconstructed subset of the array as an array.

const foo = [1, 2, 3, 4, 5];

const [a, b, ...cde] = foo;

console.log(cde); //=> [3, 4, 5]

// Can be used in other patterns
const [w, , ...[, ,z]] = foo;

console.log(z); //=> 5

The rest operator looks identical to the spread operator but the two have different functions.

In ES7, objects also support the rest operator. This allows us to grab the remaining un-deconstructed properties of the object as an object.

const foo = {a: "1", b: "2", c: "3"};
const {a, ...others} = foo;

console.log(others); //=> Object {b: "2", c: "3"}

yield statements can also be used to pass in new information to the generator.

const foo = function* (x) {
  const y = x + (yield x);

  yield y;
}

const bar = foo(5);

// outputs from 'yield x'
console.log(bar.next().value) //  5 

//outputs 'yield y' of '<x=5> + <yield x=8>'
console.log(bar.next(8).value) // 13

A little confusing right? What happens when we hit the first yield is we pass out x as value. The yield x statement then becomes whatever is passed into .next().x is still 5 but yield x is now 8.

Note: return statements in generators are not a good idea. Although they are easy to reason about for done: true, they don’t show up in for..of loops. See below

Generators can be used with for..of loops for iterating through all of the yield statements.

const foo = function* (x) {
  yield x + 1;
  yield x + 2;
  yield x + 3;

  // Any return statement here would be ignored by the for..of loop
}

for (let y of foo(6)) {
  console.log(y); //=> 7, 8, 9
}

In the for..of the variable Y is the result of the yield statements. As mentioned above, any return statement will be ignored by the loop.

JavaScript - ES6