为什么打字稿接受数字值作为类型？

Question

I had a bug in my Angular 4 project where I had declared a variable:

debounce: 300;

instead of

debounce = 300;

So of course this.debounce was undefined before the fix.

Shouldn't typescript give me an error in this case?

Answer 1

If you declare a variable with a type annotation of 300 , that means not only is the type numeric, but only the value 300 is acceptable:

var debounce: 300;

You will get an error if you attempt to assign, say, 200:

debounce = 200;

Switch on strict null checks, and the compiler will catch this kind of problem (you meant to assign the value, not a type annotation):

var debounce: 200;

// Strict null checks tells you here that you have done something strange
var x = debounce;

In this case, when you try to use the variable, strict null checks points out you never assigned a value - thus telling you that you made an annotation, not an assignment.

Answer 2

You can basically pass (almost) anything as a value type to Typescript. So if you say:

class SomeClass {
  debounce: 300 | 500 | 700; 
  constructor() {
    this.debounce = 400;
  }
}

You will get a type error, since typescript is expecting the value for debounce to be 300 , 500 or 700 , not just any number. This means that you can be more specific about type annotations.

Answer 3

That line is correct because unintentionally you have used literal-types ( for more information ):

This example helps you to understand how literal-types is used for restricting variables to a finite set of possible values :

let zeroOrOne: 0 | 1;

zeroOrOne = 0;
// OK

zeroOrOne = 1; 
// OK

zeroOrOne = 2;
// Error: Type '2' is not assignable to type '0 | 1'

Answer 4

It's not a constant declaration. You have thought declared a variable and assigned a value implicitly typed. You are wrong. It's number type declaration that accepts specific values only.

debounce: 300;

and then assign it with

this.debounce = 300;

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Typescript won't give you an error because it's syntactically correct, but runtime errors follows for undefined variables.