How do I return a tuple from a JavaScript function?

The short answer

Write the tuple type in @returns, optionally with a label for each position:

/**
 * @param {Map<string, number>} stock
 * @param {string} sku
 * @returns {[inStock: boolean, quantity: number]}
 */
export function checkStock(stock, sku) {
  const quantity = stock.get(sku) ?? 0;
  return [quantity > 0, quantity];
}

const [inStock, quantity] = checkStock(new Map([["LAMP-01", 4]]), "LAMP-01");
if (inStock) console.log(quantity.toFixed(0));

export function checkStock(stock: Map<string, number>, sku: string): [inStock: boolean, quantity: number] {
  const quantity = stock.get(sku) ?? 0;
  return [quantity > 0, quantity];
}

const [inStock, quantity] = checkStock(new Map([["LAMP-01", 4]]), "LAMP-01");
if (inStock) console.log(quantity.toFixed(0));

After destructuring, inStock is a boolean and quantity a number.

Why it works

An array literal like [quantity > 0, quantity] could be a tuple or a list, and without a hint TypeScript infers the more general one: an array whose elements are any of the values, (number | boolean)[]. The @returns type tells tsc the function returns a pair, and the literal is checked against it position by position. Labels such as inStock: arrived in TypeScript 4.0 and only name the positions.

A const cast gets the same result without writing the type, and makes the tuple read-only:

/**
 * @param {Map<string, number>} stock
 * @param {string} sku
 */
export function checkStock(stock, sku) {
  const quantity = stock.get(sku) ?? 0;
  return /** @type {const} */ ([quantity > 0, quantity]);
}

const [inStock, quantity] = checkStock(new Map([["LAMP-01", 4]]), "LAMP-01");
if (inStock) console.log(quantity.toFixed(0));

The same thing in TypeScript

The TypeScript equivalent tab writes the same tuple as the return type, : [inStock: boolean, quantity: number]. The const-cast version would be return [quantity > 0, quantity] as const;.

What goes wrong

Without either, the return type is an array, both destructured variables are number | boolean, and number methods fail (TS2339):

/**
 * @param {Map<string, number>} stock
 * @param {string} sku
 */
export function checkStock(stock, sku) {
  const quantity = stock.get(sku) ?? 0;
  return [quantity > 0, quantity];
}

const [inStock, quantity] = checkStock(new Map([["LAMP-01", 4]]), "LAMP-01");
if (inStock) console.log(quantity.toFixed(0));error TS2339: Property 'toFixed' does not exist on type 'number | boolean'.
  Property 'toFixed' does not exist on type 'false'.

Gotchas

• A const cast makes the tuple readonly. Callers cannot push to it or assign to its positions; that is usually what you want for a return value.
• The parentheses matter. /** @type {const} */ ([a, b]) is a cast; without the parentheses around the expression the comment is ignored.
• Optional positions use ?. [sku: string, quantity?: number] describes a tuple whose second element may be missing.