minlength and maxlength are attributes in HTML that control the length of input box characters. Pay attention to the correct usage and common problems. For example: Setting minlength="5" maxlength="20" can limit input characters from 5 to 20. 1. The attributes are only applicable to text type input types; 2. It is required to use it with required to avoid null values passing verification; 3. The browser prompt information may vary from device to device; 4. Common problems include spelling errors, compatibility, and script modification content resulting in invalidation; 5. In actual applications, it is recommended to use real-time counting style prompts and always perform back-end verification, while avoiding too small minlength affecting the experience.
Form verification is a very common requirement in web development, especially when it involves user input. In HTML, minlength and maxlength are two very practical properties that control the number of characters allowed to be entered in text class input boxes such as <input> or <textarea></textarea> . If used properly, it can improve the user experience and reduce the pressure on back-end verification.
Basic usage: Set the input length range
These two properties usually appear together to limit the minimum and maximum length of content input by the user. For example:
<input type="text" minlength="5" maxlength="20">
The above code means that the user must enter at least 5 characters, and must not exceed 20 characters at most. If the user tries to submit a value that does not meet the criteria, the browser will automatically prompt an error.
What should be noted is:
- These two properties are only applicable to text class input types such as
type="text"andtype="password". - They do not block user input, but are validated when submitted.
- The built-in prompt information in the browser may vary slightly depending on the system language or browser.
FAQ: Why does the settings not take effect?
Sometimes you will find that setting minlength and maxlength seems to have no effect, which is usually caused by the following reasons:
- Spelling error : For example, if written as
minLengthormax-length, the HTML attribute is case-sensitive and should be used in all lower case. - Not used with required : If you want the user to fill in and meet the length requirements, remember to add
required, otherwise the empty value will also pass the verification. - Some browser compatibility issues : Although modern browsers basically support it, additional processing may be required on some older devices.
- JavaScript dynamically modified the value : Sometimes the front-end script clears or modifys the input box content, resulting in invalid verification.
For example, the following input box requires the user to enter an 8 to 16-digit password:
<input type="password" minlength="8" maxlength="16" required>
Only by combining it in this way can we truly play a verification role.
Practical application suggestions
In actual projects, there are several tips to let you use these two properties better:
- Match style tips : You can use JavaScript to listen to input events, display the remaining word count in real time when the user enters, or mark the red reminder.
- Note the length of Chinese characters : Chinese characters are also considered as a unit of character length, and there is no need to deal with encoding issues specifically.
- The backend also needs to verify : Although the frontend has verification, never rely on it as the only line of defense, and the backend should also do the same check.
- Avoid setting minlength that is too small : For example, setting it to 3 may cause users to feel that there are too many restrictions, which will affect the experience.
To give a simple example, you can implement a real-time counting function like this:
<textarea maxlength="100"></textarea>
<div id="counter">100 words remaining</div>
<script>
const textarea = document.querySelector('textarea');
const counter = document.getElementById('counter');
textarea.addEventListener('input', () => {
const remaining = 100 - textarea.value.length;
counter.textContent = `${remaining} word remaining`;
});
</script>This way the user can clearly know how many more words they can enter.
Basically that's it. Although these two attributes seem simple, if used correctly, it can save a lot of trouble.
The above is the detailed content of HTML `minlength` and `maxlength` Attributes. For more information, please follow other related articles on the PHP Chinese website!
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