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The result is a union of string literal types: “ name” | “ salary“: type Staff = We apply the keyof operator to the Staff type, and we get a staffKeys type in return, which represents all the property names. The keyof operator takes an object type and produces a string or numeric literal union of its keys.Ī simple usage is shown below. The TypeScript handbook documentation says: Using keyof with TypeScript template string literals.Using keyof with conditional mapped types.Using keyof with TypeScript mapped types.Let’s look at how each one interacts with the keyof operator. In this article, we’re going to examine the keyof operator and how it is commonly used together with other TypeScript features to achieve better type safety with TypeScript generics, TypeScript mapped types, and TypeScript string literal types. Introduced in TypeScript 2.1, the keyof operator is used so frequently that it has become a building block for advanced typing in TypeScript. Although they are similar, keyof only works on the type level and returns a literal union type, while Object.keys returns values. In the TypeScript world, the equivalent concept is the keyof operator. In JavaScript, we often use Object.keys to get a list of property keys. How to use the keyof operator in TypeScript Yan Sun Follow I am a full-stack developer.