Javascript: Nullish Coalescing vs OR operator

First of all, what is a Nullish Coalescing operator? It is the following operator: ??

Now let’s explore what it does and what is the difference between it and the OR operator.

OR operator

The OR operator || uses the right value if the left value is falsy. A falsey value is one of the following values:

• 0, -0, 0n
• false
• "" (empty string)
• null
• undefined
• NaN

Let’s look at examples:

// when left is falsy, the right value is used
console.log(0  || "right value") 
// Output: "right value"
console.log(false || "right value") 
// Output: "right value"
console.log(""     || "right value") 
// Output: "right value"
console.log(undefined || "right value") 
// Output: "right value"
console.log(null      || "right value") 
// Output: "right value"

// but when the left value is truthy, the left value is used
console.log(12 || "right value") 
// Output: 12

Nullish coalescing

In contrast, the Nullish coalescing operator ?? uses the right value if the left value is nullish (ie, null or undefined).

For example:

// only when the left is undefined or null that the right value is used:
console.log(undefined ?? "right value") 
// Output: "right value"
console.log(null      ?? "right value") 
// Output: "right value"

// when the left value is not nullish, then the left value is used 
console.log(0  ?? "right value") 
// Output:  0
console.log("jane" ?? "right value") 
// Output: "jane"
console.log(""     ?? "right value") 
// Output: ""
console.log(true  ?? "right value") 
// Output:  true
console.log(false ?? "right value") 
// Output:  false

Most frequently, you only want the right value if the left value is null or undefined. That's what the nullish coalescing operator ?? is for.

Happy coding!
Mazen
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