Number.isFinite invalid

[lucioreyli]

TypeScript is not respecting the callback of Number.isFinite.

Example

type Product = {
    quantity?: number
};

const product: undefined | Product = {
    quantity: 34,
}

// TypeScript error! product.quantity > 3 returns error because can be undefined (?? what)
if(Number.isFinite(product?.quantity) && product.quantity > 3) { }

Explanation

Number.isFinite always return false to every variable that is not number (or better, typeof equals number). You can try all the types (including objects) and will fail if the parameter to Number.isFinite is not a number.

Expected return

// No errors is right-hand conditional because the left conditional need be a number to continue👇!!
if(Number.isFinite(product?.quantity) && product.quantity > 3) { } // ✅

Solution (partial)

To resolve this for a while, can replace the

if(Number.isFinite(product?.quantity) && product.quantity > 3) { }

to

if(typeof product?.quantity === 'number' && product.quantity > 3) { }

[whzx5byb]

Duplicate of #39090

[fatcerberus]

There are no callback functions involved in this example…