Replace a value if null or undefined in JavaScript?

In JavaScript, you can replace null or undefined values using the logical OR operator (||) or the nullish coalescing operator (??).

Syntax

// Using logical OR operator
var result = value || defaultValue;

// Using nullish coalescing operator (ES2020+)
var result = value ?? defaultValue;

Using Logical OR Operator (||)

The || operator returns the first truthy value or the last value if all are falsy:

var value = null;
console.log("The original value:", value);

var actualValue = value || "This is the Correct Value";
console.log("The replaced value:", actualValue);

// Testing with undefined
var undefinedValue = undefined;
var replacedUndefined = undefinedValue || "Default for undefined";
console.log("Replaced undefined:", replacedUndefined);

The original value: null
The replaced value: This is the Correct Value
Replaced undefined: Default for undefined

Using Nullish Coalescing Operator (??)

The ?? operator only replaces null and undefined, preserving other falsy values like 0 or empty strings:

var nullValue = null;
var undefinedValue = undefined;
var zeroValue = 0;
var emptyString = "";

console.log("null ?? 'default':", nullValue ?? "default");
console.log("undefined ?? 'default':", undefinedValue ?? "default");
console.log("0 ?? 'default':", zeroValue ?? "default");
console.log("'' ?? 'default':", emptyString ?? "default");

null ?? 'default': default
undefined ?? 'default': default
0 ?? 'default': 0
'' ?? 'default': 

Comparison

Practical Example

function getUserName(user) {
    // Using ?? to preserve empty strings but replace null/undefined
    return user.name ?? "Anonymous User";
}

var user1 = { name: null };
var user2 = { name: undefined };
var user3 = { name: "John" };

console.log("User 1:", getUserName(user1));
console.log("User 2:", getUserName(user2));
console.log("User 3:", getUserName(user3));

User 1: Anonymous User
User 2: Anonymous User
User 3: John

Conclusion

Use || when you want to replace any falsy value, or ?? when you specifically want to replace only null and undefined while preserving other falsy values.