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TypeScript Utility Types–Readonly

TypeScript provides several utility types that makes working with types easier. Today I want to take a look at the Readonly<T> type.

The Readonly<T> type makes all the properties in T become read-only, it does so by annotating each field with the readonly keyword while mapping:

It is useful to make an immutable version of a type. Here is an example:

If I load this code in the TypeScript playground, I get the following compiler errors:

I was hoping that this TypeScript code in someway was transpiled to an Object.Freeze() implementation but this turns out not to be the case. The TypeScript Readonly<T> type provides only static typing immutability(in favor of performance) whereas Object.Freeze() provides runtime immutability.

It is however possible to combine the two:

 

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