Checking instanceof with a class-CodePudding

I have the following code:

function Complex(real, imaginary) {
    let c = Object.create(Complex.methods);
    [c.real, c.imaginary] = [real, imaginary];
    return c;
}

Complex.methods = {
    toString() {return `${this.real} ${this.imaginary}i`},
    add(other) {
        if (!(other instanceof Complex)) throw new Error("Only supports complex-only addition");
        return Complex(this.real other.real, this.imaginary other.imaginary);
    }
}
let c1 = Complex(2,0);
let c2 = Complex(3,4);
console.log(c1.add(c2)   "");
// "Uncaught Error: Only supports complex-only addition",

This is occurring because c1 instanceof Complex is returning false. Why does it return false?

Comparing it with using the class keyword:

class CX {
    toString() {return "xxx"}
}
let c1 = new CX();
console.log(c1   "", c1 instanceof CX);
// xxx true

But still curious why the first one doesn't recognize the instanceof operator.

CodePudding user response：

The OP either chooses the classic (good ol' days) constructor function approach ...

function Complex(real, imaginary) {
  Object.assign(this, { real, imaginary });
}
Complex.prototype.toString = function toString () {
  return `${this.real}   ${this.imaginary}i`
};
Complex.prototype.add = function add (other) {
  if (!(other instanceof Complex)) {
    throw new Error("Only supports complex-only addition");
  }
  return new Complex(
    this.real   other.real,
    this.imaginary   other.imaginary
  );
};
let c1 = new Complex(2,0);
let c2 = new Complex(3,4);

console.log(c1.add(c2)   "");

.as-console-wrapper { min-height: 100%!important; top: 0; }

... or a class based approach ...

class Complex {
  constructor(real, imaginary) {
    Object.assign(this, { real, imaginary });
  }
  toString () {
    return `${this.real}   ${this.imaginary}i`
  }
  add (other) {
    if (!(other instanceof Complex)) {
      throw new Error("Only supports complex-only addition");
    }
    return new Complex(
      this.real   other.real,
      this.imaginary   other.imaginary
    );
  }
}
let c1 = new Complex(2,0);
let c2 = new Complex(3,4);

console.log(c1.add(c2)   "");

.as-console-wrapper { min-height: 100%!important; top: 0; }