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Unions

Unions as type property keys

type StringKeys = 'firstName' | 'lastName';
type NumberKeys = 'age';

type Person = {
    [Key in StringKeys]: string;

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Infer Function Argument Type

A function that accepts an argument of an anonymous type where hobbies is a tuple:

function stringifyPerson(person: {
    name: string;
    age: number;
    hobbies: [string, string]; // tuple
}) {
    return `${person.name} is ${person.age} years old and loves ${person.hobbies.join(" and  ")}.`;

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TypeScript infer

Extract String

type ExtractStringType<T> = T extends `${infer U}` ? U : never;
type StrType1 = ExtractStringType<'ABC'>; // 'ABC'
type StrType2 = ExtractStringType<string>; // never
type StrType3 = ExtractStringType<123>; // never

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