whole_func_string: function (el, callback) {
    var size = el.length
    var i = 0
    for (i; i < size; i++) {
      callback(el[i], i)
    }
  }
func_code_string: function (el, callback) {
    var size = el.length
    var i = 0
    for (i; i < size; i++) {
      callback(el[i], i)
    }
  }
func_documentation_string: (el →  dom, callback → function)
  - looping querySelectorAll