Learn TypeScript: The Complete Course for Beginners/

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Assertion Functions

Assertion Functions

This lesson talks about a new feature of TypeScript 3.7: the assertion function.

We'll cover the following...

Asserting untyped code

In JavaScript, you must assert the variables’ type since it is not a typed language. While the assertion function is not something you would require using purely TypeScript, it is handy to know in case you have a transpilation error on assertion.

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function showLandArea(address, x, y) { // No typê
    assert(typeof address === "string");
    assert(typeof x === "number");
    assert(typeof y === "number");
    console.log(`The house ${address.substr(10)} as an area of ${x * y} meters`)
}
showLandArea("1234 Street ABCDE", 10, 5);
// showLandArea("1234 Street ABCDE", "10", "5"); // Assertion will catch the 10 and 5 as string

With TypeScript 3.7 and above, the assert condition (the first parameter of the assert function) must be true ...