Question

I have a piece of code that tests for the existence of a variable, using an if statement like the example below. I need to do one thing if the var is set, a different thing if its not. In a certain test case, the var needed to be set to 0, but that is the same is having an unset var in JS, apparently:

var toMatch;
toMatch = 0;
if (!toMatch) {
    document.write("no");
} else {
    document.write(toMatch);
}

// html is "no"

jsFiddle

So my problem is, how do I test for a var if its value is legitimately zero. I should point out, that in my function possible values to be passed are 0-40+.

In the past I’ve used a work around like setting the initial value to a number high enough that it is not likely to be passed to the function but that seems hackey to me. Is there a better way to do it?

Answer 1

var toMatch;
toMatch = 0;
if (toMatch === 0) { // or !== if you're checking for not zero
    document.write("no");
} else {
    document.write(toMatch);
}

toMatch === 0 will check for zero.

toMatch === undefined will check for undefined

the triple equals are strict comparison operators for this sort of scenario. See this blessed question: Difference between == and === in JavaScript