A client-side script is a program that may accompany an HTML document or be embedded directly in it. The program executes on the client's machine when the document loads, or at some other time such as when a link is activated. HTML's support for scripts is independent of the scripting language.
Scripts offer authors a means to extend HTML document in highly active and interactive ways.
Scripts may accompany a form to process input as it is entered. Designers may dynamically fill out parts of a form based on the values of other fields. They may also ensure that input data conforms to predetermined ranges of values, that fields are mutually consistent, etc. Scripts may be triggered by events that affect the document, such as loading, unloading, element focus, mouse movement, etc. Those that are executed one time when the document is loaded by the user agent.