calling another function within the same parent function

ok, maybe the title is a bit confusing, but I have this issue...
I'm making my own jsextension file for a package that I'm creating for a project, the thing is I'm having this error:
js: joex-extensions.js: Javascript TypeError: this.isNumber is not a function, line 135
and the functions are like this:

var joex = new joexUtils();
joex.postln("JoeX jsexternals v0.66⍺(2020/04)");

function joexUtils(){
     this.debug = 0;
     // method to set parameter's function by variable name
    this.parameterfy = (function () {
        var pattern = /function[^(]*\(([^)]*)\)/;

        return function (func) {
            // fails horribly for parameterless functions ;)
            var args = func.toString().match(pattern)[1].split(/,\s*/);
            /*debug*/if(this.debug){this.postarr(args)};/*debug*/
            return function() {
                var named_params = arguments[arguments.length - 1];
                if (typeof named_params === 'object') {
                    var params = [].slice.call(arguments, 0, -1);
                    if (params.length < args.length) {
                        for (var i = params.length, l = args.length; i < l; i++) {
                            params.push(named_params[args[i]]);
                        }
                        return func.apply(this, params);
                    }
                }
                return func.apply(null, arguments);
            };
        };
    }());
    ...
      // check if value is a number method
    this.isNumber = function (v){
        /*debug*/if(this.debug){this.postln("joex.isNumber debug:")};/*debug*/
        var isNum = true;
        if (!Array.isArray(v)){
            isNum = (Number(v) < 0) || (Number(v) == 0) || (Number(v) > 0);
            /*debug*/if(this.debug){this.postarr([v,isNum])};/*debug*/
        }
        else {
            for (i = 0;i < v.length;i++){
                isNum = isNum && (Number(v) < 0) || (Number(v) == 0) || (Number(v) > 0);
            }
            /*debug*/if(this.debug){this.postarr(v);this.postarr([isNum])};/*debug*/
        }
        return isNum;
    }
      ...
      //clip method
    this.clip = this.parameterfy(function (value, min, max){
        /*debug*/if(this.debug){this.postarr(["joex.clip debug:",this.isNumber(value),this.isNumber(min),this.isNumber(max)])};/*debug*/
        if (this.isNumber(value) && this.isNumber(min) && this.isNumber(max)){
            /*debug*/if(this.debug){this.postarr([value,min,max])};/*debug*/
            return Math.min(Math.max(value,min),max);
        }
        else {
            this.postln("Wrong parameter(s) given to joex.clip.");
            return value;
        }
    });
...
}

so the thing is that I'm trying to use a function previously declared inside the same joexUtil function... There is no jsextension documentation so idk what is wrong, as far as I know that is possible if you declare the function previously... or am I misunderstanding it? please help!

everytime I call the function joex.clip I get the error...

PD: There are way more declarations there... the funcion this.parameterfy was copied from other place I don't remember right now...