Function.prototype.customCall = function(ctx){
    ctx.fn = this;
    let args = [...arguments].slice(1);
    let result = ctx.fn(...args);
    delete ctx.fn;
    return result
}

Function.prototype.customApply = function(ctx){
    ctx.fn = this;
    let result = arguments[1] ? ctx.fn(arguments[1]) : ctx.fn();
    delete ctx.fn;
    return result
}

Function.prototype.customBind = function(ctx){
    let that = this;
    let args = [...arguments].slice(1);
    return function(){
        that.apply(ctx, args);
    }
}