How to judge whether it is empty in real time with javascript

In web development, forms are one of the important components of business logic, and form verification is a basic security consideration. In the form, the verification of the input box is the most basic and necessary, and judging whether the content of the input box is empty is the most common verification. Here we will introduce the method of javascript to realize real-time judgment that it is not empty.

1. Real-time monitoring of the input box
   In order to achieve the effect of real-time judgment that it is not empty, we need to monitor the input box in real time through javascript. We can update the text in the input box in real time by listening to the oninput event of the input.

<input type="text" id="input" oninput="checkEmpty()">

In the above code, the oninput event is added to the input. When the event is triggered, a function named checkEmpty will be called. This function is used to determine whether there is a value in the input.

2. Verify the value in the input box
   After monitoring the input box in real time, we need to write a function checkEmpty to check whether there is a value in the input box, and change the input box according to whether there is a value style or prompt information.

function checkEmpty() {
  var inputEl = document.getElementById("input");
  if (inputEl.value.trim() == "") {
    inputEl.style.borderColor = "red";
    inputEl.nextElementSibling.innerHTML = "此项不能为空";
  } else {
    inputEl.style.borderColor = "green";
    inputEl.nextElementSibling.innerHTML = "";
  }
}

In the above code, we first obtain the DOM object of the input box, and then change the style or prompt information of the input box by determining whether the value of the input box is empty. When the input box is empty, we set the border color of the input box to red, and add the prompt message "This item cannot be empty" behind the input box; when the input box is not empty, set the border color to green , and clear the prompt information.

3. Apply the validation function to all input boxes
   Finally, we need to apply the checkEmpty function to all input boxes that require validation. In actual development, we often mark the input boxes that need to be verified with class, then use the document.getElementsByClassName() method to obtain all input boxes marked with this class, and then loop through and call the checkEmpty function again.

var inputs = document.getElementsByClassName("required");
for (var i = 0; i < inputs.length; i++) {
  inputs[i].addEventListener("input", checkEmpty);
}

In the above code, we first use the document.getElementsByClassName("required") method to obtain all input boxes that need to be verified, and then use loop traversal to add oninput events for all input boxes, and the events are triggered The checkEmpty function is executed for verification.

Summary:
Through the above three steps, we can implement a simple, javascript-based verification that the input box is not empty in real time. Of course, you may encounter more complex form validation requirements in actual development. You may wish to use the above example as a basis and make appropriate improvements based on business logic.

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