在JavaScript公共方法中获取私有方法的返回值

Let's walk through the issues in your code and get that area calculation working correctly:

What's Causing the undefined Output? #

1. You never stored the private method in your WeakMap: Your _areaCalculate WeakMap is empty for each Circle instance—you only set the _radius value in the constructor, never the method itself.
2. Incorrect method definition syntax: Using [_areaCalculate]() as a class method doesn't bind that function to your WeakMap. _areaCalculate is a WeakMap instance, not a Symbol or valid property key, so this creates a weirdly named method that's never linked to your WeakMap.
3. Accessing the radius incorrectly: this.radius doesn't exist—your radius is stored privately in the _radius WeakMap, so you need to retrieve it with _radius.get(this).

Corrected Code Option 1 (Inline Method in Constructor) #

const _radius = new WeakMap();
const _areaCalculate = new WeakMap();

class Circle {
  constructor(r) {
    _radius.set(this, r);
    // Store the private calculation method in the WeakMap for this instance
    _areaCalculate.set(this, () => {
      // Retrieve the private radius from its WeakMap
      const radius = _radius.get(this);
      return (Math.PI * Math.pow(radius, 2)).toFixed(2);
    });
  }

  areaPrint() {
    // Get the method from the WeakMap and execute it
    const calculateArea = _areaCalculate.get(this);
    console.log("The area of Circle is: " + calculateArea());
  }
}

let c = new Circle(4);
c.areaPrint(); // Output: The area of Circle is: 50.27

Corrected Code Option 2 (Reusable External Function) #

If you want to reuse the calculation logic across instances, define the function separately and bind it to the correct this context:

const _radius = new WeakMap();
const _areaCalculate = new WeakMap();

// Private function defined outside the class
function calculateCircleArea() {
  const radius = _radius.get(this);
  return (Math.PI * Math.pow(radius, 2)).toFixed(2);
}

class Circle {
  constructor(r) {
    _radius.set(this, r);
    // Bind the function to this instance and store it in the WeakMap
    _areaCalculate.set(this, calculateCircleArea.bind(this));
  }

  areaPrint() {
    // Retrieve and invoke the private method
    console.log("The area of Circle is: " + _areaCalculate.get(this)());
  }
}

let c = new Circle(4);
c.areaPrint(); // Output: The area of Circle is: 50.27

Key Fixes Explained #

• Storing the method in WeakMap: We explicitly add the calculation function to _areaCalculate for each instance in the constructor, so _areaCalculate.get(this) returns the actual method instead of undefined.
• Invoking the method: Instead of just getting the method reference, we add () to execute it and get the calculated area value.
• Accessing private radius: We use _radius.get(this) to fetch the stored private radius value, since this.radius isn't a defined property on the instance.

内容的提问来源于stack exchange，提问作者mrhrifat