[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.

巴特西

[TypeScript] Use the never type to avoid code with dead ends using TypeScript

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