checking if value is null using jQuery

This question already has an answer here:

jQuery: checking if the value of a field is null (empty) 6 answers

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The .val() always returns a string . You can check if the string is empty using:

.val().trim().length == 0

So your full condition will be:

var form_intitule = $('input[name=form_intitule]').val().trim();

if (form_intitule.length == 0) {
  alert('fill the blank');
  return false;
}

---

我会建议一个更安全的方法

var formElement = $('input[name=form_intitule]');
if ($(formElement).length == 0 || $(formElement).val().trim().length == 0) {
   console.log("element does not exist OR value is empty");
}

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