[TypeScript] Use the never type to avoid code with dead ends using TypeScript
Example 1: A never stop while loop return a never type.
function run(): never {
while(true){
let foo = "bar";
}
}
Example 2: Never run If block
const foo = ; if(foo !== ) {
let bar: never = foo;
}
You can use this to do exhaustive checks in union types.
For example, let's say you have a variable returned from the server that can be a string or a number. You can easily add code that handles different cases using the JavaScript typeof operator. You can add an additional else, and assign the variable to a never to ensure that all types were eliminated.
declare var foo:
| string
| number; if(typeof foo === "string") {
/* todo */
} else if (typeof foo === "number"){
/* todo */
} else {
const check: never = foo;
}
Later, if you need to add another type to the union, for example, a Boolean, you will now get nice errors at all the places where the new type was not handled, because only a never is assignable to a never. Now, if you go ahead and add another typeof to handle this new case, the error goes away.
[TypeScript] Use the never type to avoid code with dead ends using TypeScript的更多相关文章
- TypeScript开发环境搭建(Visual studio code)
使用Visual Studio Code搭建TypeScript开发环境 1.TypeScript是干什么的 ? TypeScript是由微软Anders Hejlsberg(安德斯·海尔斯伯格,也是 ...
- [TypeScript] Infer the Return Type of a Generic Function Type Parameter
When working with conditionals types, within the “extends” expression, we can use the “infer” keywor ...
- [TypeScript] Union Types and Type Aliases in TypeScript
Sometimes we want our function arguments to be able to accept more than 1 type; e.g. a string or an ...
- [TypeScript] Use the TypeScript "unknown" type to avoid runtime errors
The "any" type can be very useful, especially when adding types to an existing JavaScript ...
- [转]VS Code 扩展 Angular 6 Snippets - TypeScript, Html, Angular Material, ngRx, RxJS & Flex Layout
本文转自:https://marketplace.visualstudio.com/items?itemName=Mikael.Angular-BeastCode VSCode Angular Typ ...
- 【区分】Typescript 中 interface 和 type
在接触 ts 相关代码的过程中,总能看到 interface 和 type 的身影.只记得,曾经遇到 type 时不懂查阅过,记得他们很像,相同的功能用哪一个都可以实现.但最近总看到他们,就想深入的了 ...
- TypeScript之定义类型 ( type )
键值对结构的对象 export type ValidationErrors = { [key: string]: any }; 联合类型(union type) export type HttpEve ...
- [TypeScript] Generic Functions, class, Type Inference and Generics
Generic Fucntion: For example we have a set of data and an function: interface HasName { name: strin ...
- [TypeScript] Define a function type
type DigitValidator = (char) => boolean; -]{}/.test(char); export const digitValidators: {[key: s ...
随机推荐
- NIO专栏学习
http://blog.csdn.net/column/details/12993.html
- thinkphp动态注册路由
thinkphp动态注册路由 一.总结 1.thinkphp使用路由步骤:a.config配置文件中开启路由 b.Route类的rule方法创建路由(在Routephp中)Route::rule(' ...
- Serializable中的serialVersionUID到底有啥用
最近在研究跨进程通信的问题,于是又再一次研究了,我们熟悉而又陌生的Serializable接口. 那么好,做过Java开发的朋友肯定对这个接口不陌生吧,Java中就是通过这个接口,来实现了序列化和反序 ...
- COGS——C2274. [HEOI 2016] tree
http://www.cogs.pro/cogs/problem/problem.php?pid=2274 ★☆ 输入文件:heoi2016_tree.in 输出文件:heoi2016_tre ...
- hdu5389
题意:给你n个人每一个人手里有一个id,然后给你两个数a和b.让你把n个人分为两组.条件是 一组人手里的id和等于a 另一组人的id和等于b,这里的和是指加起来之后对9取余,假设sum等于0 则sum ...
- iOS_04_数据类型、常量、变量
一.数据 1.什么是数据 * 生活中时时刻刻都在跟数据打交道,比如体重数据.血压数据.股价数据等.在我们使用计算机的过程中,会接触到各种各样的数据,有文档数据,图片数据,视频数据,还有聊天QQ产生的文 ...
- LeetCode Algorithm 07_Reverse Integer
Reverse digits of an integer. Example1: x = 123, return 321Example2: x = -123, return -321 click to ...
- promis:异步编程
promise对象用于延迟计算和异步计算:一个promise对象代表着一个还未完成,但预期将来完成的操作 Image.png Image.png 打印结果如下: <!DOCTYPE html&g ...
- 加固linux
http://linoxide.com/linux-command/password-aging-secure-linux-access/
- mangodb学习0.1 概念
摘自菜鸟教程