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TypeScript Tutorial

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TS Keyof

keyof is a TypeScript keyword used to extract the key types from an object type.

keyof with explicit keys

keyof is a TypeScript keyword used to extract the key types from an object type.

Example

interface Person {
  name: string;
  age: number;
}
// `keyof Person` here creates a union type of “name” and “age”, other strings will not be allowed
function printPersonProperty(person: Person, property: keyof Person) {
  console.log(`Printing person property ${property}: “${person[property]}”`);
}
let person = {
  name: “Max”,
  age: 27
};
printPersonProperty(person, “name”); // Printing person property name: “Max”

keyof with index signatures

keyof can also be used with index signatures to extract the index type.

type StringMap = { [key: string]: unknown };
// `keyof StringMap` resolves to `string` here
function createStringPair(property: keyof StringMap, value: string): StringMap {
  return { [property]: value };
}

 

 

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