Error: 'callback is not a function' when using function.apply()

Why is this not working as expected?

The error message indicates that typeof(callback) is undefined.

function A(a, callback)
{
    document.write(typeof(callback));  
    callback();  
    return a;  
}

function Run(func, args)
{

    return func.apply(this || window, args || [
        function () { 
            document.write("blah")
        }
    ]);

}


Run(A, [1]);

Interestingly, it works properly when not using function.apply:

function Run2(func, arg)
{

    return func(arg,
        function () {
            document.write("blah")
        }
    );

}

Run2(A, 1);

I'm still getting familiar with JavaScript, so please bear with me.

Answer №1

When using the apply method, remember that the first argument represents the scope while the second argument should be an array of arguments. It seems like you are passing this correctly, but with args in Run(A,[1]);, you are only providing a single argument (1) which will be assigned to parameter a. However, the function itself is missing.

If args is not defined, you are essentially creating an array with a single argument [ function ()... ], which once again will be assigned to a.

It appears that you might be trying to merge or concatenate two arrays, but keep in mind that || is used as a comparison operator or for assignment with the logical OR operation.

You can try this:

func.apply(this || window, args.concat([function () { document.write("blah")}]));

or

args.push(function () { document.write("blah")});
func.apply(this || window, args);

Answer №2

The problem is occurring within the A function and specifically with the callback variable. To avoid encountering this error, make sure to define the A function in the following way:

function A(a, callback) {
    var type = typeof callback;
    document.write(type);
    if(type === "function") callback(); // Make sure that callback is a function
    return a;
}

Answer №3

apply() utilizes the first argument as the reference for this in the function call and uses the second argument as the parameters for that call. Therefore, Run2 is invoking A like this (where <func> represents your anonymous function):

A(1, <func>);

In contrast, Run only passes a single argument in the argument array, 1, and executes it in this way:

A(1)

The preferred action would be to substitute your existing Run with the following approach (as per my understanding):

function Run(func,args)
{
    args.push(function () { document.write("blah")});
    // args contains [1, function(){...}]
    return func.apply(this||window, args);
}

Answer №4

In order for your Run() function to properly utilize A(), it requires an Array length of 2 because two arguments are needed. However, you currently only have 1 argument in [1]. To correct this issue and ensure that the function works as intended with [1, func], you can implement the following solution:

// utilizing the keyword 'this'
function Run(func, args){
  args.push(func);
  return func.apply(this, args || [function(){document.write("blah")}]);
}

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