Can not call a method from inside another method in Javascript

Question

Check my code below, I am just lost here why am I getting this error. Any suggestion please. Here I have made a class test and added two methods check and nextfn. I am calling check from nextfn.

var test=function(){}

test.prototype.check=function()
{
  console.log("hello from checking");
}

test.prototype.nextFn=function(){

  check();

  console.log("Hello from nextfn");
}

var t=new test();
t.nextfn();

The error is

Uncaught ReferenceError: check is not defined(…)

Now consider another scenario;

test.prototype.anotherFn=function()
{
    var p=new Promise(function(){
         this.check();
    })
}

Now also getting same error;

Uncaught ReferenceError: check is not defined(…)

When calling

var t=new test();
t.anotherFn();

to expand on @Igor comment, the check method was put on the prototype of test. nextFn is part of the same prototype, this is a keyword referring to the parent/prototype. — kyler
– kyler, Commented Feb 8, 2017 at 14:16

Jonathan.Brink · Accepted Answer · 2017-02-08 14:28:14Z

6

The check function is on the prototype of the test object.

When you invoke nextFn like this:

t.nextfn();

The ensuing scope will be bound to the t instance of "type" test. Access within nextfn to test's prototype will be available via this.

So access check using this:

this.check();

This stuff can get surprisingly confusing. A good reference is this book.

====

For your second scenerio, the problem is that you are trying to invoke this from within a function that has it's own scope.

Scope in JavaScript is generally not block scoped, but rather function scoped. There is a lot more to it, and I would recommend reading a tutorial on closures to get a more rounded description, but for now, try this instead:

test.prototype.anotherFn=function()
{
    var self = this; // save reference to current scope
    var p=new Promise(function(){
         self.check(); // use self rather than this
    })
}

edited Feb 8, 2017 at 14:28

answered Feb 8, 2017 at 14:14

Jonathan.Brink

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2 Comments

KOUSIK MANDAL Over a year ago

Worked well, now added another scenario in my question; can you please clarify this one also please. I understand that it is now going to the scope of p. But how to call the function check then?

Jonathan.Brink Over a year ago

@KOUSIKMANDAL I updated my answer for the second part of the question

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