Question

I would like to find a way to programmatically (i.e. by writing code) find for what values of x we have the expression x == null evaluating to true.

Answer 1

It is impossible to (without prior knowledge of at least the basic JavaScript rules) be able to programmatically determine all values of x for which x == null is true. However, the following should show a pattern from which a heuristic can be derived:

var v = [null, undefined, false, true, -1, 0, 1, "", " ", "0", {}, []]
for (var i = 0; i < v.length; i++) {
   var x = v[i]
   alert(x + " == null? " + (x == null))
}

(This particular test case does cover all the times when it would be true.)

Similar tests can be done for == false, etc.

Happy coding.

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See Ray Toal’s answer for more suggestions of test values.