Many developers use client-side JavaScript to test and validate data that customers enter into forms. Why send data to the server if you can determine its validity at the browser? Unfortunately, most of the JavaScript validation schemes I've seen have been very inflexible. For instance, many scripts validate data when the customer submits the form, but only a few provide instant feedback while the user enters data. And many scripts are customized to a specific form so that each change to the form requires more work on the developer's part, adding and deleting code from a single, monolithic validation function.
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