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.

Answer 2

You can test using typeof (among other things):

if (typeof x == 'undefined') x = y;

Another approach is to test strict equality against undefined:

if (x === void 0) x = y

In this case we use void 0 as a safety since undefined can actually be redefined.

Answer 3

You can do this quite quickly with just a

x === undefined && (x = 0);

Answer 4

For ES6 users you can simply do:

x ?? 0

?? is a Nullish coalescing operator:

a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

Answer 5

You can use double tilde (~~) bitwise NOT operator.

var a; // undefined
a = ~~a
console.log(a) // 0

It works as a substitute for (+) which works for undefined too:

var a = "10";
a = ~~a
console.log(a) // 10

Answer 6

You can use a combination of typeof and isNaN functions to achieve your desired behavior. Here's an example:

if (typeof x === "undefined" || isNaN(x)) {
  x = defaultValue;
}

This code checks if the value of x is undefined or NaN, and if so, replaces it with a default value. Note that the isNaN function is necessary to avoid replacing NaN values, since NaN is not equal to itself and therefore cannot be compared using the usual equality operators.

Alternatively, you can use the Number.isNaN function, which is a newer and more reliable way to check for NaN values:

if (typeof x === "undefined" || Number.isNaN(x)) {
  x = defaultValue;
}

This code behaves the same way as the previous example, but uses the Number.isNaN function instead of isNaN. Note that Number.isNaN is only available in newer JavaScript versions (ES6 and later).

I hope this helps!