What is true in JavaScript? It depends on how you ask.

X === true is true only if and only if X is a Boolean variable set to true.

(What, you’ve never heard of the “triple-equals” operator? It’s actually called the “identity” operator, and X === Y when typeof(X) == typeof(Y) && X == Y. There’s also X !== Y, which is the same as !(X === Y). You’re probably familiar with the typeof operator – it returns “number”, “string”, “boolean”, “object”, “function”, or “undefined”. Notice that there’s no “integer” or “real”, so it is unsurprising (but dangerous, given the unpredictability of floating-point math) that 1 + 1 === 0.5 + 1.5. Anyway, back to what is true…)

X ? true : false is true most of the time. It’s only false when X is the number 0, or the number NaN, or the empty string, or the false Boolean, or the null object, or undefined. (This is the same logic used by the if and while statements.)

Now, equality gets really confusing. X == true whenever X can be directly converted to the Boolean true. The following meet that criteria: the true Boolean, the number 1, and strings that convert to the number 1, such as “1”, “1.0”, “1E0”, “0.1E1”, “ +1.0 ”, etc. Note that leading and trailing whitespace is ignored! Surprisingly, strings like “true” or “True” do not meet the criteria – so "1" == true, but "true" != true.

In the case of equality, we must also talk about what is false. X == false whenever X can be directly converted to the Boolean false: the false Boolean, the number 0, and strings that convert to the number 0, such as “0”, “0.0”, “ -0E1”, etc. But wait; there’s more! The empty string (“”) is also “equal to” false, as well as any string that contains only whitespace! So " \n\t\r\u2002" == false! Who knew? However, a few things that you’d think might be “equal to” false aren’t: "false" != false, NaN != false, null != false, and undefined != false.

What's particularly confusing is considering which of the following similar statements are true for any given X:

• X == true
• X != false
• X ? true : false

If X is "1", they are all true. If X is "", none are true. But if X is null, only the second is true. If X is " ", only the last is true. And if X is "2", the last two are true. The funny thing is that it really does work as you’d expect, most of the time. The subtleties of JavaScript…

Disclaimer: My experience is actually with Microsoft’s JScript, particularly as it is implemented inside Internet Explorer. I presume that this is all true for all JavaScript engines, but I haven’t actually tested it.

Update: I replaced the second to last paragraph with something that is hopefully more interesting and more accurate. (Thanks, Bradley!)