ICode9

精准搜索请尝试: 精确搜索
首页 > 其他分享> 文章详细

[Typescript] Tuple type usage example

2022-07-26 15:03:06  阅读:177  来源: 互联网

标签:Typescript const string second Error usage outcome type name


function flipCoin(): "heads" | "tails" {
  if (Math.random() > 0.5) return "heads"
  return "tails"
}

function maybeGetUserInfo():
  | ["error", Error]
  | ["success", { name: string; email: string }] {
  if (flipCoin() === "heads") {
    return [
      "success",
      { name: "Mike North", email: "mike@example.com" },
    ]
  } else {
    return [
      "error",
      new Error("The coin landed on TAILS :("),
    ]
  }
}

 

Working with union types

Let’s continue with our example from above and attempt to do something with the “outcome” value.

const outcome = maybeGetUserInfo()
 
const [first, second] = outcome

first  // const first: "error" | "success"
second // const second: Error | {
       //    name: string;
       //    email: string;
       // }

We can see that the autocomplete information for the first value suggests that it’s a string. This is because, regardles of whether this happens to be the specific "success" or "error" string, it’s definitely going to be a string.

The second value is a bit more complicated — only the name property is available to us. This is because, both our “user info object, and instances of the Error class have a name property whose value is a string.

 

What we are seeing here is, when a value has a type that includes a union, we are only able to use the “common behavior” that’s guaranteed to be there.

 

Narrowing with type guards

Ultimately, we need to “separate” the two potential possibilities for our value, or we won’t be able to get very far. We can do this with type guards.

const outcome = maybeGetUserInfo()
const [first, second] = outcome

if (second instanceof Error) {
  // In this branch of your code, second is an Error
  second
} else {
  // In this branch of your code, second is the user info
  second
}

TypeScript has a special understanding of what it means when our instanceof check returns true or false, and creates a branch of code that handles each possibility.

It gets even better…

 

Discriminated Unions

const outcome = maybeGetUserInfo()
if (outcome[0] === "error") {
  // In this branch of your code, second is an Error
  outcome // const outcome: ["error", Error]
} else {
  outcome 
  /*
  const outcome: ["success", {
    name: string;
    email: string;
  }]*/
}

TypeScript understands that the first and second positions of our tuple are linked. What we are seeing here is sometimes referred to as a discriminated or “tagged” union type.

标签:Typescript,const,string,second,Error,usage,outcome,type,name
来源: https://www.cnblogs.com/Answer1215/p/16521008.html

本站声明: 1. iCode9 技术分享网(下文简称本站)提供的所有内容,仅供技术学习、探讨和分享;
2. 关于本站的所有留言、评论、转载及引用,纯属内容发起人的个人观点,与本站观点和立场无关;
3. 关于本站的所有言论和文字,纯属内容发起人的个人观点,与本站观点和立场无关;
4. 本站文章均是网友提供,不完全保证技术分享内容的完整性、准确性、时效性、风险性和版权归属;如您发现该文章侵犯了您的权益,可联系我们第一时间进行删除;
5. 本站为非盈利性的个人网站,所有内容不会用来进行牟利,也不会利用任何形式的广告来间接获益,纯粹是为了广大技术爱好者提供技术内容和技术思想的分享性交流网站。

专注分享技术,共同学习,共同进步。侵权联系[81616952@qq.com]

Copyright (C)ICode9.com, All Rights Reserved.

ICode9版权所有