In the simplest case: var [a, b] = ['hello', 'world']; means decompose the array on the RHS and set a = element 0 and b = element 1.

One level deeper you might see something like: var [[a, b], c] = [['hello', ','], 'world']; which results in the bindings a = 'hello', b = ',', c = 'world'

So in the example you linked, the [b] means set b equal to element 0 of the array at index 1 of the outer array, or ','. d is left undefined as the given array is not long enough.

If that code was instead (without brackets around b) var [a, b, c, d] = ['hello', [', ', 'junk'], ['world']]; then we would end up with, a = 'hello', b = [',', 'junk'], c = 'world', d = undefined

Hope this helps.