Q:

Javascript - Replace undefined with 0

I have a variable that gets defined by user input. I want to replace it's value only if it's undefined. But not if it's NaN. How can I do it? 
I tried doing x || 0 but that also replaces NaN values.

A:

You can do a strict comparison to the undefined value.
if (x === undefined) {
    x = 0;
}

Naturally you'll want to be sure that x has been properly declared as usual.
If you have any sensitivities about the undefined value being plagued by really bad code (overwritten with a new value), then you can use the void operator to obtain a guaranteed undefined.You can do a strict comparison to the undefined value.
if (x === void 0) {
    x = 0;
}

The operand to void doesn't matter. No matter what you give it, it'll return undefined.
These are all equivalent:
if (x === void undefined) {
    x = 0;
}

if (x === void "foobar") {
    x = 0;
}

if (x === void x) {
    x = 0;
}

Ultimately if someone squashed undefined locally (it can't be squashed globally anymore), it's better to fix that bad code.

If you ever want to check for both null and undefined at the same time, and only those value, you can use == instead of ===.
if (x == null) {
    x = 0;
}

Now x will be set to 0 if it was either null or undefined, but not any other value. You can use undefined in the test too. It's exactly the same.
if (x == undefined) {
    x = 0;
}

From your question, it seems a little bit like you're specifically looking for number elements, even NaN. If you want to limit it to primitive numbers including NaN, then use typeof for the test.
if (typeof x !== "number") {
    x = 0;
}

However, you'll lose numeric strings and other values that can successfully be converted to a number, so it depends on what you ultimately need.

