JavaScript - Logical OR Assignment Operator

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javascript

What is Logical OR Assignment?

The logical OR assignment (||=) is a logical operation that takes two values: a target variable and a source value.

Here’s how it works:

  • If the original value of the target variable is truthy (i.e., it’s not false, 0, null, undefined, NaN, or an empty string ""), then nothing happens. The target variable keeps its original value.
  • If the original value of the target variable is falsy (i.e., it is one of the following: false, 0, null, undefined, NaN, or an empty string ""), then the target variable is assigned the value of the source value.

Examples

  • Example with a number:
let num = 0;
num ||= 100;

console.log(num); // ouput: 100
  • Example with a string:
let str = "";
str ||= "another string";

console.log(str); // output: another string
  • Example with an object:
let obj = null;
obj ||= { greeting: "Hello" };

console.log(obj); // output: { greeting: "Hello" }
  • Example with an array:
let arr = undefined;
arr ||= [1, 2, 3];

console.log(arr); // Output: [1, 2, 3]

Recap

The logical OR assignment is a handy tool that assigns a new value to a variable if its original value is falsy. For instance, if a string is empty or a number is zero, the operator will assign a new value to them/ However, if these variables already hold truthy values, their values will remain unchanged.

In JavaScript, empty arrays ([]) and empty objects ({}) are seen as “truthy”. So, they won’t get a new value with the ||= operator. You can check this by using Boolean(), both will return true.

In essence, if the original value of a variable is either null, undefined, false, 0, or an empty string (""), then the variable will be assigned a new value.

Resources

MDN Docs - Logical OR assignment (||=)

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