Question

Question inside the description

function Parent(){
    this.alertParent(){
        alert("Parent alert");
    }

    function child(){
        // how can I call to this.alertParent() from here without passing any  
        // parameters?
    }
}

Answer 1

The title of your question is confusing. The informal term “parent” function is rather used for the calling function.

In your case, you have two functions inside a constructor function and you just want to call one from the other. Specifically, you want to call a “public” method from a “private” method (I put these terms in quotes since JavaScript does not support visibility and these are workaround to achieve the same).

Just keep a reference to the current instance:

function Parent(){
    var self = this;
    this.alertParent = function() {
        alert("Parent alert");
    }

    function child() {
        self.alertParent();
    }
}

child closes over all variables in the context it is defined, so it as access to self. this of course changes ^[MDN].

Instead of creating a closure, you can also pass the instance explicitly to child, using either .call() ^[MDN] or .apply() ^[MDN].

So your function definition stays

function child() {
    this.alertParent();
}

and when you call the function, you call it, e.g. with child.call(this) if you know that this refers to your instance (instead of this it can be any other variable).