打字稿：您可以在类的私有属性中使用构造函数属性吗？

Question

Is it bad practice to use a constructor properties of a class within a variable? I tried this and was surprised const name = new Name(time, course); worked, although I guess it makes sense since an instance of Eng needs those constructor properties to be made, then they'd be available to use elsewhere.

import { Name } from './Name'

class Eng {
   private _name = new Name(time, course);

   private constructor(time: String, course: String) {} 

   method1(){
      let results = name.convertTrials();

      return results;
   }
}

Answer 1

Your code example actually has type errors:

class Eng {
   private _name = new Name(time, course);
   // Cannot find name 'time'.(2304)
   // Cannot find name 'course'.(2304)

   private constructor(time: string, course: string) {} 
}

time and course are not in scope as far as typescript is concerned. If you want to use constructor parameters, they must be in constructor body.

---

What may be confusing you is that, when compiled, the code runs fine. That is because non-static class properties are moved into the constructor for you, since that is where they would need to be in plain JS.

So your compiled class looks like:

// js
class Eng {
    constructor(time, course) {
        this._name = new Name(time, course);
    }
}

Which should work without a problem.

But it is a problem for Typescript, which believes that the constructor arguments are not in scope outside of the constructor.

---

If you don't need the arguments:

class Foo {
    constructor() {}
}

class UseFoo {
   private foo = new Foo()
   private constructor() {} 
}

Then there isn't anything wrong with your approach.

See Playground