首页 > 代码库 > [TypeScript] Use the never type to avoid code with dead ends using TypeScript
[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 = 123;if(foo !== 123) { 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