Calling property of One Function From Inside Another Javascript

Question

I'm using functions as representations of classes.

function ClassOne()
{
  this.doFunc = function() {
    console.log("doFunc output");
  }
}

function ClassTwo()
{
  this.one = ClassOne();
  console.log(this.one); // == undefined ???

  this.init = function() {
    this.one.doFunc();
  }

  this,init();
}

ClassTwo();

But I'm struggling to use one function inside another.

For example, the above code returns "Cannot read property 'doFunc' of undefined"

Why is this.one == undefined?

Why is this.one == undefined?: Because you are executing the function and the function has no return statement. Therefore, it returns undefined and assigns that to your this.one property. — Randy Casburn
– Randy Casburn, Commented Dec 6, 2018 at 4:47
You need to use new ClassOne() is you want it to make an instance for you. — Mark
– Mark, Commented Dec 6, 2018 at 4:48

CertainPerformance · Accepted Answer · 2018-12-06 04:49:22Z

If you assign to a property of this inside a function, generally that indicates that the function is intended to be called as a constructor with new, eg:

this.one = new ClassOne();

Otherwise, if the function doesn't return anything... then, well, nothing gets returned (=> undefined)

function ClassOne()
{
  this.doFunc = function() {
    console.log("doFunc output");
  }
}

function ClassTwo()
{
  this.one = new ClassOne();
  console.log(this.one);

  this.init = function() {
    this.one.doFunc();
  }

  this.init();
}

ClassTwo();

Or, you can have ClassOne explicitly return an object with a doFunc property, allowing it to be called without new:

function ClassOne()
{
  return {
    doFunc() {
      console.log("doFunc output");
    }
  }
}

function ClassOne() {
  return {
    doFunc() {
      console.log("doFunc output");
    }
  }
}

function ClassTwo() {
  this.one = ClassOne();
  console.log(this.one);

  this.init = function() {
    this.one.doFunc();
  }

  this.init();
}

ClassTwo();

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