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Data-entry validation checks whether information entered into an application follows rules the application expects before it accepts or processes that information. It can flag missing, malformed, or out-of-range values and help a person correct them. Passing validation does not prove that a value is factually true.
What data-entry validation means
An application defines the information it needs and the acceptable rules for each field. Validation compares entered data with those rules. For example, a form might require a value, limit its length, accept only a certain type, or restrict it to a range or set of allowed choices.
MDN Web Docs defines input validation as “the practice of checking that any input that your website accepts is what you expect.” MDN Web Docs: Input validation
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What rules can validation check?
Syntax and format
Syntactic validation checks whether data has an expected structure or form. A field may require a date in an accepted format, an email-like address, or a value with a prescribed pattern. HTML forms provide constraints such as type and pattern for some of these checks.
Requiredness, type, and length
A field can be mandatory, limited to a particular input type, or constrained by a minimum or maximum length. HTML attributes include required, minlength, and maxlength. These rules help catch omissions and values that do not fit the field’s expected shape.
Ranges and allowed values
Semantic validation checks whether a value is acceptable in the application’s context. A numeric field might have minimum and maximum values or an increment rule; a selection may need to match one of the options the application allows. HTML constraints include min, max, and step, while more complex rules can be implemented in application code or on the server. MDN Web Docs: Client-side form validation
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Where validation happens
In the browser
Client-side validation runs in the browser, often as a person fills out or submits a form. It can provide fast feedback and catch routine mistakes before a request is sent. HTML form constraints can support this behavior, and custom client-side code can add checks.
On the server
Server-side validation checks submitted data before the application processes it. This is essential: browser checks can be bypassed by changing the page or sending a crafted request. The server must not assume that client-side validation ran or succeeded. The WHATWG HTML Standard also makes clear that client-side validation does not replace server verification. WHATWG: HTML Standard, forms
Use both for different purposes
Browser checks improve the user’s experience; server checks are the authoritative gate before processing. The same important rules should be enforced on the server even when the browser also checks them. MDN Web Docs: Input validation
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to make validation useful to people
- Identify required fields clearly, rather than leaving people to discover requirements only after submission.
- Explain the specific problem in text and give a practical correction path—for example, say which field needs attention and what kind of value is accepted.
- Accept legitimate ways people express the same information. Postal codes, for instance, are not numeric in every country; telephone numbers may use different separators.
- Make error notifications accessible and noticeable, and associate them with the relevant field where possible.
- For consequential or irreversible actions, consider letting people review or confirm the submitted details.
These practices align validation with the real data and the people providing it, instead of rejecting valid information simply because it does not match a needlessly narrow pattern. W3C WAI: Validating Input W3C WAI: Forms Tutorial
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Validation can show that an entry meets specified rules: for example, that a required field is filled in and that its value falls within an allowed range. It cannot, by itself, establish that the entry reflects reality. Applications that need factual confirmation require an appropriate separate process; a successful format or range check alone is not proof.
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