Introduction

I oftentimes find that I have forgotten the object rest properties syntax.

The following is a collection of examples to which I expect to be able to refer in order to remind myself of the rules of the syntax.

Examples

Flat Object

Create an object.

// A flat object. Imagine a letter bank (i.e. inventory) in the game, "Words With Friends."
> let bank = {a: 1, b: 2, c: 3};

Order of Properties

Create a new object, based upon bank, using rest properties syntax.

> let newBank = {a: 4, ...bank};
> console.log(newBank);
Object {
  "a": 1,
  "b": 2,
  "c": 3
}

Notice: The a: 4 did not affect the result. Notice where the a: 4 is in our assignment (hint: it's before the rest properties operator).

Now, move the a: 4 to after the rest properties operator and repeat the experiment:

> newBank = {...bank, a: 4};
> console.log(newBank);
Object {
  "a": 4,  // <--
  "b": 2,
  "c": 3
}

Notice: This time, the a: 4 affected the result!

Adding a Property

Notice: In the following examples, we are specifying a key that does not exist on the bank object (defined above).

> newBank = {d: 4, ...bank};
> console.log(newBank);
Object {
  "a": 1,
  "b": 2,
  "c": 3,
  "d": 4  // <--
}

> newBank = {...bank, d: 4};
> console.log(newBank);
Object {
  "a": 1,
  "b": 2,
  "c": 3,
  "d": 4  // <--
}

Notice: In both examples, the d: 4 affected the result (i.e. a additional property was created). Whether we put the d: 4 before or after the rest properties operator, the resulting object still contained the additional property.

References

Related Babel plugin: http://babeljs.io/docs/plugins/transform-object-rest-spread/