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I using javascript to check the length of height input by the user and but the problem I am facing is that even javascript just check the first number of the input and throws an error. Just for eexample if I have to input 45 in the height, I get an error height must be between 6-36 just after entering 4 it doesnt let me enter 5 and when i try to remove move and it input becomes empty It again throws an error height must be between 6-36. Please help me find the problem.

<input type="number" id="message1" name="height" oninput="function_two()">

function function_two() {
    var FrameHeight = document.getElementsByName('height')[0].value;
    if (FrameHeight <= 36 && FrameHeight >= 6) 
        return true;
    else 

    alert("Height must be between 6-36");
  }
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  • 2
    Use onchange, or a form and a submit event listener. But please, please, please don't use inline event attributes. They are the devil. Commented Nov 20, 2018 at 22:49
  • Try the onblur event instead of oninput. That will wait until the input loses focus. (Or onchange like Tiny Giant said... that will wait until the input is changed and loses focus which is probably better...) Commented Nov 20, 2018 at 22:50
  • How is the code supposed to know how many digits you want to add? I suggest you either use onblur as the event, that is fired when user leaves the input, or use a slider or (there is another html5 element that I don't remember), to restrict the value to your limits Commented Nov 20, 2018 at 22:50
  • What if you used a text input rather than a number? Commented Nov 20, 2018 at 22:50
  • 1
    @TinyGiant well, it's a part of the window object ..... :-) Commented Nov 20, 2018 at 22:57

7 Answers 7

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How is the code supposed to know how many digits you want to add? So that it triggers the validation? At the moment, the function is called every time the user gives an input, that is every time a user types.

I suggest you either use change as the event, that is fired when user leaves the input after it changes it, or change the type to range. This way you don't need to do the validation.

I would advice you use <input type="range" min="6" max="36" step="1" /> if you can. Note that it is not supported by all browsers as it is a HTML5 element.

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2 Comments

Wasn't me who down voted, but if you've got a browser that isn't HTML5 compliant at this point, that's your problem.
Yes, I have pointed that out. But so is the input type="number" for that matter
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Sounds like you have a few problems and therefore a couple possible solutions

If you want the error to appear after your first character but still want to be able to keep entering characters until the requirements are met, try displaying the error by adding it to your page somewhere instead of using alert.

If you want to only check the value when someone is done typing, you can use onchange instead of oninput, though this means that the user will have to defocus the input.

If you want to check the value when someone is done typing but without having to defocus the input you should look into using a debounce function. Underscorejs has a good one or you can write your own.

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The function is ok. 4 is less than 6 therefore it throws an error. The best way forward is to run function after user has finished typing. To do this, edit the html section to onchange.

<input type="number" id="message1" name="height" onchange="function_two()"> 


function function_two() {
 var FrameHeight = this.value;
  if( trim (value ) == '' ){
     return false;
   }
  if (FrameHeight < 6 || FrameHeight > 36) { 
     alert("Height must be between 6-36");
   } 
}
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8 Comments

onblur fires even if there is no change. I fail to see how that is "The best way" to proceed.
You can return false if trim value equals ''
I fail to see how that would solve the problem of calling the function when there is no change. The change event is dispatched when the input blurs and the value has changed, no point in reinventing the wheel. This is literally not "The best way forward". 100%
The input type is number. Spaces aren't allowed in the field in the first place, so trim() is not needed.
@scott i never had trim in the first place. I like your answer. You are using change which i suggested earlier and someone voted me down. Then i added the return false... :)
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You are using the wrong event (input), which fires as any input is given to the field. Use the change event, which fires when the value changes and the field loses the focus.

Additionally, separate your JavaScript from your HTML. Do your event handling with modern, standards-based practices, rather than with inline HTML event attributes, which should not be used.

See the comments below for other adjustments to the solution that make the code more efficient and/or update it to modern standards.

// Get your DOM reference just once. .querySelector() is preferred
// over .getElementsByName, .getElementsByTagName, .getElementsByClassName
// as the former returns a static node list and the latter(s) return 
// live node lists that hurt performance.
let nameInput = document.querySelector("input[name='height']");

// And set up event handlers in JavaScript, not HTML
nameInput.addEventListener("change", rangeCheck);

// Name functions with descriptive names as to what they do.
// Don't use the word "function" in a function name.
function rangeCheck() {
    // In a DOM event handler, you can just use "this" as a reference
    // to the DOM element that triggered the event.
    var FrameHeight = this.value;
    // Just test for the bad values and act accordingly
    if (FrameHeight < 6 || FrameHeight > 36) {
      alert("Height must be between 6-36");
    }
}
<input type="number" id="message1" name="height">

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3 Comments

I think after input is complete is before we leave the field, change feels too late. How about keyup?
@Fanky You make a good point, but doing that would then have the data checked upon each keystroke and what if the user needs to enter a multiple digit number?
true, I ended up by combining validating max on keyup and the rest on change. The max on keyup also prevents from writing third digit (by returning to data-last attribute saved on beforeinput). You could even set an inactivity timer to solve the trouble that we can't ever know when input is finished.. But that seem overkilly :)
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You are validating each keystroke entered, so you may want to combine a combination of oninput and onblur to validate the entire value and the keystrokes as they happen.

Try something like this:

<html>
<head>
<style>
    input.invalid {
        background-color: red;
    }
</style>
<script>
    function validateInput(inputElement) {
       var frameHeight = inputElement.value
       if (isValidHeight(frameHeight)) {
           inputElement.className = ""
       } else {
          inputElement.className = "invalid"
       }
    }

    function validateFrameHeight(inputElement) {
        var frameHeight = inputElement.value
        if(!isValidHeight(frameHeight)) {
            alert("Height must be between 6-36");
        }
    }

    function isValidHeight(frameHeight) {
        return frameHeight <= 36 && frameHeight >= 6
    }
</script>
</head>
<body>

    <input class="invalid" type="number" id="message1" name="height" oninput="validateInput(this)" onblur="validateFrameHeight(this)">

</body>

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3 Comments

This seems like an even worse idea. I seriously do not get the aversion to using the proper event (change), and the need to reinvent the wheel for absolutely no reason.
Agree and disagree. I agree that change is a better event for validation than blur. However I disagree that oninput is a bad way to get real-time feedback on your currently entered data.
But the problem is that they don't need real time feedback, the need feedback on change. Hacking oninput and onblur together to make a fragile substitute for onchange is not a good alternative to onchange.
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@TinyGiant thankyou. it worked

onchange="function_two()"
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I would think focus out event would help https://api.jquery.com/focusout/

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