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Edvins Antonovs

Exploring TypeScript: typeof and keyof

If you're a software engineer most likely you come across the TypeScript language and its powerful type system. Today, we will explore two TypeScript features that can make your code more robust and expressive: typeof and keyof.


At its core, typeof in TypeScript allows you to capture the type of a variable or object. This can be incredibly handy when you want to create new variables that inherit the type of an existing one. For example:

1const originalVariable = {
2 name: 'Edvins',
3 age: 30,
6const newVariable: typeof originalVariable = {
7 name: 'Antonovs',
8 age: 31,

In this example, newVariable now has the same type as originalVariable. This ensures that if you ever update the structure of originalVariable, TypeScript will catch any mismatches in newVariable during development, reducing the chance of runtime errors.


keyof, on the other hand, is all about working with object keys. It allows you to create types that represent all possible keys of an object. Here's how it looks:

1const person = {
2 name: 'Edvins',
3 age: 30,
6type PersonKeys = keyof typeof person; // "name" | "age"

In this case, PersonKeys is a type that can only be one of the keys in the person object. This comes in handy when you want to write functions or utilities that work with object keys in a type-safe way.

Putting it all together

Now, let's combine the power of typeof and keyof to create a versatile function:

1function getProperty<T, K extends keyof T>(obj: T, key: K): T[K] {
2 return obj[key];
5const person = {
6 name: 'Edvins',
7 age: 30,
10const name: string = getProperty(person, 'name'); // Works perfectly
11const invalidKey: boolean = getProperty(person, 'invalidKey'); // TypeScript error!

In a nutshell, typeof helps mirror types effortlessly, while keyof empowers us to work with object keys in a type-safe manner.

By combining these tools, as seen in getProperty<T, K extends keyof T>, you can craft functions that fetch properties in a way that maintains type safety. These features bolster your TypeScript toolkit, ensuring your code remains robust and predictable.

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