打字稿不强制执行或检查返回类型

Question

A bug in our code ended up being caused by the below. The output of the method was being compared against 0 and returning false. The method is explicitly declaring a number return type, but allowing a string to be returned. I would have epxected this to be flagged as an error at some point between compiler and lint, but is never caught.

Comparison:

if(routerIdParameter() === 0) {
  console.log('They match');
} else {
  console.log('They do not match');
}

Method:

public routerIdParameter(): number {
  if (this.route.snapshot.paramMap.has('id')) {
    return this.route.snapshot.params['id'];
  } else {
    return 0;
  }
}

Obviously the problem line is return this.route.snapshot.params['id'] . params type is [key: string]: any so it can't know the type, and there is where I assumed type checking would complain. There is the easy fix of prefixing it with + but I'm more concerned with my gap in understanding as to why it is allowed.

Any explanation to why this is happening or how to make it error would be appreciated as I'm concerned there could be more of this throughout our app.

Identified in typescript 2.3.4 but also reproduced in 3.1.6

this.route is of type ActivatedRoute imported from @angular/router

Answer 1

any by definition is assignable from any type and assignable to any type. There is no compiler setting to make this stricter.

Where possible you should prefer unknown (introduced in typescript 3.0) which is assignable from any type but not assignable to any type (read here for more info on any vs unknown ).

As to how you can avoid this behavior, I would suggest looking into the tslint rule no-unsafe-any