Problem with Javascript 'document' is not defined [closed]

HERE is the code

var nic = document.getElementsByClassName('formulario__input'); 
for(var i = 0; i< nic.length; i++) {
    nic[i].addEventListener('Keyup', function(){
        if(this.value.length>=1){
            this.nextElementSibling.classList.add('fijar');
        }else{
            this.nextElementSibling.classList.remove('fijar');
        }
    });
}