how to do deep equal on sparse array?

test.js

var require('assert')

var a = []

// make sparse array
a[20] = true

var b = []

// try copying a's items into b with splice
b.splice.apply(b, [0, 0].concat(a.slice(0, 21)))

assert.deepEqual(a, b) // throws?

console.log(b)
/*
[ undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
  true ]
*/

console.log(a)
/*
[ , , , , , , , , , , , , , , , , , , , , true ]
*/

console.log(a[0] === b[0])

/* 
true
*/

Ahh, I've got it.

b has numeric keys 0 - 20. a does not. assert.deepEqual must do something related to the keys.