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Firefox with Firebug<br />
From the console you can call functions or add new functions.
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<div class="content wysiwyg-content">
What tools/plugins can I modify and run js in from the browser (when web page is loaded from a server). For example, trigger an existing function or modify it. Add a new function.
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What tools/plugins can I modify and run js in from the browser (when web page is loaded from a server). For example, trigger an existing function or modify it. Add a new function.

Firefox with Firebug
From the console you can call functions or add new functions.

JS testing tool

JavaScript is a dynamic, object-based language commonly used for client-side scripting in web browsers. Recently, server side JavaScript frameworks have also emerged. JavaScript runs on nearly every operating system and  in almost every mainstream web browser.