TypeScript Variable Type: never

Definition of never

neverOther types (including nulland undefined) sub-type, from the representative value does not occur.

There are usually two manifestations of never:

• Throw an exception

  // 返回值为 never 的函数可以是抛出异常的情况
  function error(message: string): never {
        
        
      throw new Error(message);
  }
  

• Cannot execute to the end point

  // 返回值为 never 的函数可以是无法被执行到的终止点的情况
  function loop(): never {
        
        
      while (true) {
        
        }
  }
  

Features of never

• never Variables of type can be assigned to variables of any type;

  let num: number;
  
  num = (() => {
        
        
      throw new Error('exception');
  })();
  

• neverTypes of variables can only be neverthe type of assignment variables.

  let invalid1: never;
  let invalid2: never;
  
  invalid1 = "Hello World!"; // error, string 类型不能转为 never 类型
  invalid2 = (() => {
        
        
      throw new Error('exception');
  })();