?? vs || in JavaScript: A Crucial Distinction

While both the nullish coalescing operator (??) and the logical OR operator (||) are used for providing default values in JavaScript, they have distinct behaviors that can significantly impact your code's logic.

Understanding Falsy and Nullish Values

Before diving into the differences, let's clarify these terms:

  • Falsy values: In JavaScript, these are values that evaluate to false in a Boolean context. They include 0, '', NaN, null, undefined, and false itself.
  • Nullish values: A subset of falsy values, specifically null and undefined.

The Logical OR Operator (||)

The logical OR operator returns the first truthy value in an expression or the last value if all are falsy.

Example:

JavaScript

let name = '';
let greeting = name || 'Hello, World!'; // greeting will be 'Hello, World!'

In this case, name is an empty string (falsy), so the OR operator returns the right-hand side, which is 'Hello, World!'.

The Nullish Coalescing Operator (??)

The nullish coalescing operator only returns the right-hand side if the left-hand side is either null or undefined. It preserves other falsy values.

Example:

JavaScript

let age = 0;
let message = age ?? 'Age is unknown'; // message will be 0

Here, age is 0, which is falsy but not nullish, so the nullish coalescing operator returns age itself.

When to Use Which

  • Use || when:
  • You want to provide a default value for any falsy value (including 0, '', etc.).
  • You're unsure if the value might be falsy.
  • Use ?? when:
  • You specifically want to handle null or undefined values.
  • You want to preserve other falsy values.

A Practical Example

Consider a function that calculates a discount based on a customer's loyalty level:

JavaScript

function calculateDiscount(loyaltyLevel) {
  return loyaltyLevel ?? 0.1; // Default discount is 10%
}

Use code with caution.

Using ?? ensures that customers without a loyalty level (represented by null or undefined) get the default discount, while those with a loyalty level of 0 still get their specific discount.

Key Takeaways

  • The || operator is more inclusive, considering all falsy values.
  • The ?? operator is more specific, targeting only null and undefined.
  • Understanding the difference is crucial for writing precise and reliable code.

By carefully choosing between ?? and ||, you can enhance your code's clarity and prevent unexpected behavior.

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