Javascript - Replace undefined with 0

Question

I have a variable that gets defined by user input. I want to replace its 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.

Answer 1

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.