How to set a default value

Short answer

The ?? operator returns the right value only if the left one is null or undefined:

const limit = undefined;
 
console.log(limit ?? 10);

Output:

Function parameters

A default value can be written right in the parameter list:

const greet = (name = 'guest') => `Hello, ${name}!`;
 
console.log(greet());
console.log(greet('Anna'));

Output:

Hello, guest!
Hello, Anna!

In destructuring

When unpacking an options object, set defaults for each field:

const createButton = ({ text = 'OK', color = 'blue' } = {}) => `${text} (${color})`;
 
console.log(createButton({ text: 'Save' }));
console.log(createButton());

Output:

Save (blue)
OK (blue)

Assigning only if empty

??= writes a value only if the property is empty:

const options = { retries: 0 };
options.retries ??= 3;
options.timeout ??= 5000;
 
console.log(options.retries, options.timeout);

Output:

0 5000

Common mistake

The || operator replaces all falsy values, including 0 and an empty string:

const volume = 0;
 
console.log(volume || 50);
console.log(volume ?? 50);

Output:

50
0

Thanks for reading!

Short answer

Function parameters

In destructuring

Assigning only if empty

Common mistake

Functions used in this recipe

Thanks for reading!