How to destructure and initializing missing, nested fields in the same expression?

Question

I've been trying to figure out if nested ES6 destructuring supports this functionality. I would like to assign a as an empty object if it doesn't exist, and subsequently assign it's property name b to a scope variable b.

var {
  a = {},
  a: {
    b = 2,
  },
} = {};

VM40:4 Uncaught TypeError: Cannot destructure property `b` of 'undefined' or 'null'.
    at <anonymous>:6:5

It seems I can't really nest destructuring without assuming the objects format. The following works, but I'm looking for a more concise method where you can easily see the nested layout of the original object.

var {
  a = {},
} = {};

var {
  b = 2,
} = a;

// works as expected
// starts to look crazy when 'forking' branches that may or may not exist

Answer 1

You're looking for

var {
  a: {
    b = 2,
  } = {},
//  ^^^^
} = {};

The default initialiser comes after the target (which is the inner destructuring expression here). If you are looking to initialise both the variables a and b, there's not much else you can do other than having two assignments:

var {
  a = {},
} = {},
{
  b = 2,
} = a;

This might look cleaner with two var declarations like you did. If you insist on doing it in one nested expression, you can do

var {
  a = {}, // short for `a: a = {}`
  a: {
    b = 2,
  } = a, // use either `a` or another empty object as the default
} = {};

but I consider that pretty unreadable.