TypeScript-All-In-One(A.R)

Assertion Functions

In TypeScript, assertion functions are functions that indicate the verification of a specific condition based on their return value. In their simplest form, an assert function examines a provided predicate and raises an error when the predicate evaluates to false.

function isNumber(value: unknown): asserts value is number {
    if (typeof value !== 'number') {
        throw new Error('Not a number');
    }
}

Or can be declared as function expression:

type AssertIsNumber = (value: unknown) => asserts value is number;
const isNumber: AssertIsNumber = value => {
    if (typeof value !== 'number') {
        throw new Error('Not a number');
    }
};

Assertion functions share similarities with type guards. Type guards were initially introduced to perform runtime checks and ensure the type of a value within a specific scope. Specifically, a type guard is a function that evaluates a type predicate and returns a boolean value indicating whether the predicate is true or false.

This differs slightly from assertion functions,where the intention is to throw an error rather than returning false when the predicate is not satisfied.

Example of type guard:

const isNumber = (value: unknown): value is number => typeof value === 'number';