In JavaScript, how to get the parameters of a prototype method in a constructor through a prototype chain?

JavaScript prototype chain and constructor parameter access

In JavaScript, efficient utilization of prototype chains is key. This article discusses how to access the parameters of a prototype method indirectly in the constructor. It is impossible to access the parameters of the prototype method directly in the constructor, because the prototype method is called only after the object is instantiated, and the execution time of the two is different.

For example:

 function Abc(k) {
  this.val = k this.q; // this.q is undefined, an error or the result is uncertain}

Abc.prototype.add = function(q) {
  return this.val q; 
};

In the Abc constructor, the parameter q of the add method cannot be directly obtained.

Solution: Indirect access

We can simulate parameter passing by updating instance properties in the prototype method.

 function Abc(k) {
  this.val = k;
  this.addParams = {}; // Used to store parameters of add method}

Abc.prototype.add = function(q) {
  this.addParams = q; // Store parameters to instance attribute return this.val q;
};

let m = new Abc(1);
m.add(2);
console.log(m.addParams); // 2

Now, the parameter q of the add method is stored in addParams property of instance m , thereby accessing indirectly outside the constructor. If q is an object, a reference to that object is stored.

More flexible solution: use closures

For more elegant handling, closures can be used:

 function Abc(k) {
  this.val = k;
  let addParams; // Use closure variable to store parameters this.add = function(q) {
    addParams = q; // Store parameters to closure variable return this.val q;
  };

  this.getAddParams = function() { // Method to get parameters return addParams;
  };
}

let m = new Abc(1);
m.add(2);
console.log(m.getAddParams()); // 2

This method uses the closure variable addParams to store the parameters of add method and provides the getAddParams method to access the parameter, avoiding direct modification of instance properties.

In summary, while the parameters of the prototype method cannot be accessed directly in the constructor, similar effects can be achieved indirectly by updating instance properties in the prototype method or using closures, and keeping the code neat and maintainable. Which method to choose depends on the specific requirements and code style.

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