When to use ?? and || in JavaScript and TypeScript

In this short post, I want to share a simple way to remember when to use ?? and || when checking values in JavaScript and TypeScript code.

Note: You can read this post on Medium too

When to use ?? when checking a value?

The ?? is an or operator where the left side must be empty before returning the right side.

The question mark ? is like asking yourself, “Does the value exist?”

var ?? varMustBeEmptyToReturnThisVar

Examples

Suppose you get an object where some properties are optional.

const obj0 = { required: 0 };
const { optional } = obj0;
console.log(optional ?? 'no optional property exists');
// returns 'no optional property exists'

Be careful when the property returns a falsy value.

const obj1 = { required: '' };
const { required } = obj1;
console.log(required ?? 'no required property exists');
// returns ''

When to use || when checking a value?

The || is an or operator where the left side must be falsy before returning the right side.

The vertical bar | is like putting up a barrier where only truthy values can stay on the left side.

var || varMustBeFalsyToReturnThisVar

Example

Suppose you get an object where some properties are falsy.

const obj2 = { required: '' };
const { required } = obj2;
console.log(required || 'required property is falsy');
// returns 'required property is falsy'

Conclusion

I sometimes misuse the ?? and || and thought I should write this reminder to myself. Hopefully that helps you too.

Before you go

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