Setting the value of a text field using Javascript

(tip: press the + or - button)

The code

function trim(str) {
  return str.replace(/^\s+|\s+$/g, '');
}


function isEmpty(str) {
  str = trim(str);
  return ((str == null) || (str.length == 0))
}


function isDigit(c) {
  return ((c >= "0") && (c <= "9"))
}


function isInteger(str) { 
  var i;
  for (i = 0; i < str.length; i++) {
  var c = str.charAt(i);
  if (!isDigit(c)) return false;
  }
  return true;
}


function monthModify(txtElement, btnElement) {
 
  switch(btnElement.name) {
    case 'increase':
      if(isEmpty(txtElement.value)) {
        txtElement.value = '1';
      } else if(isInteger(txtElement.value)){
        txtElement.value ++;
      } else {
        alert('The value you are trying to increment is not an integer');
        txtElement.value = '';
      }
    break;

    case 'decrease':
      if(isEmpty(txtElement.value)) {
        txtElement.value = '0';
      } else if(isInteger(txtElement.value)){
        if(txtElement.value > 0) {
          txtElement.value --;
        } else {
          alert('The minimum value is zero for this field');
        }
      } else {
        alert('The value you are trying to decrement is not an integer');
        txtElement.value = '';
      }
    break;

    default:
  }
}

function initForm(oForm, element_name, init_txt) {
  frmElement = oForm.elements[element_name];
  frmElement.value = init_txt;
}

function showFormData(oForm) {
  var msg = "The data that you entered for the form : \n";
 
  for (i = 0; i < oForm.length, oForm.elements[i].getAttribute("type") !== 'button'; i++) {
    msg += oForm.elements[i].tagName + " with 'name' attribute='" + oForm.elements[i].name + "' and data: ";
    if(oForm.elements[i].value == null || oForm.elements[i].value == '') {
    msg += "NOT SET \n";
    } else {
      msg += oForm.elements[i].value + "\n";
    }
  }

  alert(msg);
}


function clearFieldFirstTime(element) {
  if (element.counter==undefined) {
  element.counter = 1;
  }

  else {
  element.counter++;
  }

  if (element.counter == 1) {
  element.value = '';
  }
}



Download the code here: javascript-form-value-example-2.zip

Back to article: set the value of a form element using Javascript