Determining type conversion programmatically

I would like to find a way to programmatically (i.e. by writing code) find for what values of x we have the express开发者_Go百科ion x == null evaluating to true.

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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.

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In the absence of knowing at least something about how JavaScript performs type conversions the way these conversions are applied over the operator == you would have to test every possible value against null, and the number of possible values are unlimited, so what you ask cannot be done.

With a little bit of knowledge you can break down the world of all possible JavaScript expressions into the following categories:

• undefined
• null
• true
• false
• 0
• negative finite numbers
• positive finite numbers
• negative infinity
• positive infinity
• NaN
• empty string
• a string full of whitespace
• a string with at least one non whitespace character
• an empty object
• an object with some properties

Test a representative value from each class against null and see what you get.

I assume from the way the question was phrased that you know the exact section in the ECMA-262 specification that defines, precisely, the semantics of ==. ( Section 11.9.3 of the 5.1 spec )