GM_openInTab (or any other GM_ function) is not defined?

Question

When my GM script does this:

var curTab = GM_openInTab(url);

it results in a \'GM_openInTab is not defined\' JavaScript error

Answer 1

In order to use any of the GM_ functions, you must set a matching @grant directiveDoc (As of Greasemonkey version 2.0Release notes)

For example:

// ==UserScript==
// @name     _YOUR_SCRIPT_NAME
// @include  http://YOUR_SERVER.COM/YOUR_PATH/*
// @grant    GM_openInTab
// ==/UserScript==

var curTab  = GM_openInTab ("http://pwnthemall.com/");

Note that this has the side effect of switching Greasemonkey's sandbox back on. See also:

• Error: Permission denied to access property 'handler'
• How to access `window` (Target page) objects when @grant values are set?

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Tampermonkey emulates most of this behavior as of version 3.9Release notes. But the current version (3.9.202) still attempts to guess appropriate values if @grant is not specified, so you won't necessarily see an error (yet).
Always use @grant anyway, for maximum compatibility and to future-proof your code.