The following would assign to the address foo in obj if and only if it has not already been assigned.
var obj = {} var foo = Math.random() obj[foo] ?= 1 The same can be said for: var undef undef ?= 1 Which could both be transpired to type obj[foo] === 'undefined' && obj[foo] = 1 or type a === 'undefined' && undef = 1 respectively.
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