Search code examples
javascriptecmascript-6destructuring

Function parameter object destructuring results in undefined variables

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.

Solution

  • 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 }}))