Syntax

Use value of x if it not null or undefined, otherwise use value of y.

const z = x ?? y

That is equivalent to:

const z = (x !== null && x !== undefined) ?
    x :
    y;

Note that this does not check if x is truthy.

If use double pipe syntax instead, then if x is an empty string, zero, null, undefined, etc. then y will be used.

const z = x || y

Examples

var one = 1
var two = 2
var nullValue = null
var udfValue = undefined

one ?? two
// one

nullValue ?? two
// two
udfValue ?? two
// two

You could use this syntax to get values in a nested object or call function in the last part.