Javascript invoke constructor function once when object instantiated with new

Question

I am instantiating an object in javascript using a constructor. Like so:

var Constructor = function(){
    this.property1 = "1";    
}
var child = new Constructor();
console.log(child) // Constructor {property1: "1"}

I would like a method to be invoked once whenever a child object is instantiated via the new keyword. I would like this method to only be available to the Constructor.

This is what I have come up with so far:

var Constructor = function(property2){
    this.property1 = "1";
    (function(){ this.property2 = property2}).call(this);
}
var child = new Constructor("2")
console.log(child) // Constructor {property1: "1", property2: "2"}

Is this the correct way to approach this problem in Javascript? Is there a cleaner or more robust way that I could approach this problem?

I assume the actual function you want to call is defined somewhere else? Because as it is, it would be much cleaner to simply inline this.property2 = "2";. Are you asking whether f.call(this) is the correct way of calling a function and setting this to a specific value? If yes, then yes. — Felix Kling
– Felix Kling, Commented Dec 30, 2016 at 1:20
@FelixKling I am just using that function as an example, I would replace that code with other functionality that would mutate the object based upon parameters entering the Constructor function. I will edit the question to reflect this — Gerard Simpson
– Gerard Simpson, Commented Dec 30, 2016 at 1:26

Oriol · Accepted Answer · 2016-12-30 01:35:00Z

1

What you are doing seems kind of useless because you could directly use

var Constructor = function(property2) {
  this.property1 = "1";
  this.property2 = property2;
};

But if your constructor does complex things and what you want is splitting them into parts for better abstraction, then personally I would take these parts outside in order to have a cleaner constructor:

var Constructor = (function() {
  function someLogic(instance, value) {
    instance.property2 = value;
  }
  return function Constructor(property2) {
    this.property1 = "1";
    someLogic(this, property2);
  };
})();

edited Dec 30, 2016 at 1:35

answered Dec 30, 2016 at 1:29

Oriol

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4 Comments

Gerard Simpson Over a year ago

I have edited my question to reflect that my constructor would be doing more complex things. Thank you

Jaromanda X Over a year ago

No you haven't made it clear in any edit that the function would be any more complex

Barmar Over a year ago

@GerardSimpson The only edit you made was to add a parameter to the constructor. What does that have to do with the complex function you want to call?

Gerard Simpson Over a year ago

If you can abstract the idea that the function is doing more than setting a simple variable, you will be able to get to the crux of the issue. Thanks

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