Set a variable if undefined in JavaScript

Question

Yes, it can do that, but strictly speaking that will assign the default value if the retrieved value is falsey, as opposed to truly undefined. It would therefore not only match undefined but also null, false, 0, NaN, "" (but not "0").

If you want to set to default only if the variable is strictly undefined then the safest way is to write:

var x = (typeof x === 'undefined') ? your_default_value : x;

On newer browsers it's actually safe to write:

var x = (x === undefined) ? your_default_value : x;

but be aware that it is possible to subvert this on older browsers where it was permitted to declare a variable named undefined that has a defined value, causing the test to fail.

Logical nullish assignment, ES2020+ solution

New operators are currently being added to the browsers, ??=, ||=, and &&=. This post will focus on ??=.

This checks if left side is undefined or null, short-circuiting if already defined. If not, the right-side is assigned to the left-side variable.

Comparing Methods

// Using ??=
name ??= "Dave"

// Previously, ES2020
name = name ?? "Dave"

// Before that (not equivalent, but commonly used)
name = name || "Dave" // name ||= "Dave"

Basic Examples

let a       // undefined
let b = null
let c = false

a ??= true  // true
b ??= true  // true
c ??= true  // false

// Equivalent to
a = a ?? true

Object/Array Examples

let x = ["foo"]
let y = { foo: "fizz" }

x[0] ??= "bar"  // "foo"
x[1] ??= "bar"  // "bar"

y.foo ??= "buzz"  // "fizz"
y.bar ??= "buzz"  // "buzz"

x  // Array [ "foo", "bar" ]
y  // Object { foo: "fizz", bar: "buzz" }

??= Browser Support Sept 2021 - 90%

??= Mozilla Documentation

||= Mozilla Documentation

&&= Mozilla Documentation

The 2018 ES6 answer is:

return Object.is(x, undefined) ? y : x;

If variable x is undefined, return variable y... otherwise if variable x is defined, return variable x.

ES2020 Answer

With the Nullish Coalescing Operator, you can set a default value if value is null or undefined.

const setVariable = localStorage.getItem('value') ?? 0;

However, you should be aware that the nullish coalescing operator does not return the default value for other types of falsy value such as 0 and ''.

However, do take note of the browser support. You may need to use a JavaScript compiler like Babel to convert it into something more backward compatible. If you are using Node.js, it has been supported since version 14.

I needed to "set a variable if undefined" in several places. I created a function using @Alnitak answer. Hopefully it helps someone.

function setDefaultVal(value, defaultValue){
   return (value === undefined) ? defaultValue : value;
}  

Usage:

hasPoints = setDefaultVal(this.hasPoints, true);

It seems more logical to check typeof instead of undefined? I assume you expect a number as you set the var to 0 when undefined:

var getVariable = localStorage.getItem('value');
var setVariable = (typeof getVariable == 'number') ? getVariable : 0;

In this case if getVariable is not a number (string, object, whatever), setVariable is set to 0

In our days you actually can do your approach with JS:

// Your variable is null
// or '', 0, false, undefined
let x = null;

// Set default value
x = x || 'default value';

console.log(x); // default value

So your example WILL work:

const setVariable = localStorage.getItem('value') || 0;