[TypeScript] Model Alternatives with Discriminated Union Types in TypeScript-优快云博客

本文介绍如何利用TypeScript的标记联合类型定义有限的对象形态集合，并通过实例演示了如何创建一个通用的Result类型，该类型包含成功和失败两种情况。此外，还展示了如何运用这些类型来模拟不同的支付方式。

TypeScript’s discriminated union types (aka tagged union types) allow you to model a finite set of alternative object shapes in the type system. The compiler helps you introduce fewer bugs by only exposing properties that are known to be safe to access at a given location. This lesson shows you how to define a generic Result<T> type with a success case and a failure case. It also illustrates how you could use discriminated unions to model various payment methods.

In the example, we make Result type restrict the return type, if success, it should return value, if not, return error prop.

type Result<T> =
  | { success: true; value: T }
  | { success: false; error: string };

function tryParseInt(text: string): Result<number> {
  if (/^-?\d+$/.test(text)) {
    return {
        success: true,
        value: parseInt(text, 10)
    };
  }
  return {
    success: false,
    error: "Invalid number format"
  };
}

const result = tryParseInt("42");

if (result.success) {
 result; // refer to success case only
 console.log(result.value)
} else {
  result; // refer to error case only
}

interface Cash {
kind: "cash";
}

interface PayPal {
kind: "paypal";
email: string;
}

interface CreditCard {
kind: "creditcard";
cardNumber: string;
securityCode: string;
}

type PaymentMethod = Cash | PayPal | CreditCard;

function stringifyPaymentMethod(method: PaymentMethod): string {
switch (method.kind) {
    case "cash":
        return "Cash";
    case "paypal":
        return `PayPal (${method.email})`;
    case "creditcard":
        return "Credit Card";
}
}

const myPayment = {
kind: "paypal",
email: "typescript@egghead.io"
}

console.log(stringifyPaymentMethod(myPayment))