[Typescript] Specify Exact Values with TypeScript’s Literal Types

A literal type is a type that represents exactly one value, e.g. one specific string or number. You can combine literal types with union types to model a finite set of valid values for a variable. In this lesson, we explore the all kinds of literal types in TypeScript:

• String literal types
• Numeric literal types
• Boolean literal types
• Enum literal types

First String literal types:

let autoComplete: "on" | "off" | "ON" | "OFF";
autoComplete = "On" // case sensitive, compiler error

Number literal types:

type NumberBase = 2 | 8 |10 | 16;
let base: NumberBase;
base = 2;
base = 4; // error 

Boolean literal types:

let autoFocus: true = true;
autoFocus = false; // error

Enum literal types:

enum Protocols {
    HTTP,
    HTTPS,
    FTP
}

type HyperTextProtocol = Protocols.HTTP | Protocols.HTTPS;

let protocol: HyperTextProtocol;
protocol = Protocols.HTTP;
protocol = Protocols.HTTPS;
protocol = Protocols.FTP; // error

原文地址：https://www.cnblogs.com/Answer1215/p/9676021.html