How do I use callback to access this variable?

Question

I've called myFunc inside myFunc2. Now what I want is to print var2 when I call myFunc2 using callback concept. I know something is wrong. The code I've tried isn't working.

It is showing this error:

Uncaught ReferenceError: newfunc is not defined(…)

How can I fix it?

CODE:

<html>
<head>
    <title></title>
    <script>
        function myFunc(parameter){
            var var2="another thing";
            newfunc(var2);
        }
        function myFunc2(){
            var var1="Something";
            myFunc(newfunc(some));
            console.log(var1);
            console.log(var2);
        }
    </script>
</head>
<body>
<button onclick="myFunc2()">BUTTON</button>
</body>
</html>

EDIT:

According to comments, I didn't defien newfunc(). Now I've defined it. But it's not working.

<script>

        function myFunc2(){
            var var1="Something";
            myFunc(function newfunc(some){
                console.log(var2);
            });
            console.log(var1);
        }
        function myFunc(parameter){
            var var2="another thing";
            newfunc(var2);
        }
    </script>

what about `newfunc`? also in myFunc2 declaration, the `some` is not initialized, this is another problem. — marmeladze, Nov 28 '16 at 10:45
This is what I don't understand. I know the code is wrong. I tried this example to understand this thing: http://stackoverflow.com/questions/40838085/how-to-get-the-value-of-str-variable ( luschn's answer) — Vikas Kumar, Nov 28 '16 at 10:47
If you pass `myFunc(newfunc);` then you need to define newfunc and then you need to call `parameter(var2)` in myfunc — mplungjan, Nov 28 '16 at 10:47
@VikasKumar I have tried to make my answer little descriptive but I strongly suggest you to refer http://stackoverflow.com/questions/9596276/how-to-explain-callbacks-in-plain-english-how-are-they-different-from-calling-o — Rajesh, Nov 28 '16 at 11:12

score 0 · Answer 1 · answered Nov 28 '16 at 10:49

0

This is the way it works:

  var a = function myFunc(parameter){
                var var2="another thing";
                parameter.call();
          }

    a(function(){console.log("the function parameter")})

answered Nov 28 '16 at 10:49

kimy82

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score 0 · Answer 2 · answered Nov 28 '16 at 10:53

I am not that sure what you are trying to achieve. Is it something like this?

 var var2;
 function myFunc(parameter){
    var2="another thing";
    return var2;
 }
 function myFunc2(){
     var var1="Something";
     myFunc(var1);
     console.log(var1);
     console.log(var2);
}

<button onclick="myFunc2()">BUTTON</button>

You can check this Fiddle

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

Function cannot share variable defined in their individual scope but they can share their values using arguments and return value. This is how 2 function talk.

Also remember, when you add () after a function name, it is called immediately. If you need to pass callback, you will have to pass reference (functionName).

You can refer following post: Passing parameters to a callback function for more information.

function newfunc(str){
  return "The test String is: " + str;
}

function myFunc(callback) {
  var var2 = "another thing";
  // return value
  return callback(var2);
}

function myFunc2() {
  var var1 = "Something";
  // Accept value that is returned and save it in local variable
  // Also adding "()" after function name will call it and pass its return value as argument.
  // If you even need to pass reference of a function with callback, use `.bind(context, args)`
  var var2 = myFunc(newfunc);
  console.log(var1);
  console.log(var2);
}

<button onclick="myFunc2()">BUTTON</button>

Snippet 2 explanation

First, defining function inside () will make it an expression and its reference will not be preserved. Due to this, your function is still not accessible.

Second, when you pass a function as a callback, it reference is shared and is available by argument name. In your case parameter. So instead of newfunc(var2) do parameter(var2)

Third, var2 does not exists inside newfunc. You are sharing value using argument (some). Use this to log.

function myFunc2() {
  var var1 = "Something";
  // Here newfunc will be ddestroyed after the call as it is wrapped inside ()
  myFunc(function newfunc(some) {
    console.log(some);
  });
  console.log(var1);
}

function myFunc(parameter) {
  var var2 = "another thing";
  parameter(var2);
}

myFunc2();

How do I use callback to access this variable?

3 Answers3

Snippet 2 explanation

Linked