Function parameter object destructuring results in undefined variables

Question

I am running Node v6.6.0, which has support for destructuring function arguments:

function foo ({ a: { b }}) {
  // stuff
}

Suppose I want to destructure and access both a and b. Sadly the following doesn't seem to work:

function foo ({ a: { b }}) {
  return [a, b]
}
foo({ a: { b: 123 }})
// ReferenceError: a is not defined!

Is this bug in Node or is this the expected behavior for ES6? Shouldn't both a and b be defined in the function? If not, why does destructuring have the effect of un-defining a base property name (a)?

Is there a way I can use parameter destructuring to get both a and b defined in the function? I'm explicitly trying to avoid manually destructuring them.

a merely designates what property of the parameter b is to be taken from. It is not defined or accessible as a parameter. If you want to access a, you will have to extract b yourself within the function. — user663031
– user663031, Commented Aug 1, 2017 at 16:05

Yury Tarabanko · Accepted Answer · 2017-08-01 16:05:15Z

4

Is this bug in Node or is this the expected behavior for ES6?

This is expected behaviour. Because {a: {b}} doesn't bind a as a name it just indicates you want to access a property of the destructured object.

You could use the following.

function foo ({ a, a: {b} }) {
  return [a, b]
}
console.log(foo({ a: { b: 123 }}))

answered Aug 1, 2017 at 16:05

Yury Tarabanko

45.1k10 gold badges88 silver badges99 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

GladstoneKeep Over a year ago

Ah, so simple! That didn't even cross my mind as a possibility. Thanks!

Collectives™ on Stack Overflow

Function parameter object destructuring results in undefined variables

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related