Using the typeof Operator in TypeScript

In Javascript, we already have a vanilla typeof operator which can be used to find the type of anything:

let x = "hello world";
console.log(typeof x); // Returns "string"

With TypeScript being a strongly typed language, typeof takes on a slightly different meaning. Although all the ’ typeof functionality remains the same, it also gets some additional, useful features. Let's look at how typeof works in TypeScript.

How typeof works in TypeScript

The most basic application of typeof in TypeScript is the creation of new basic types. If we are defining our own , we can use typeof to copy the type of an existing item. A simple example where we take a number, and create a custom type off the back of it looks like this:

let x = 1234;

// Custom type aNumber
type aNumber = typeof x;

let x = 1234;
let y = "string";

// Custom type aNumber
type myType = {
    name: typeof y,
    age: typeof x,
}

As you can see, typeof basically gives us a way to differentiate between the value and type of an existing object. It can also be combined with ReturnType quite intuitively, to get the returned value of a function, to ensure type consistency when expecting values from functions:

function myFunction(x: string, y: string) {
    return { 
        firstName: x,
        lastName: y
    }
}
type nameType = ReturnType<typeof myFunction>;

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