GM Common API (GMC) is a library designed for easily adding Greasemonkey 4 compatibility to existing userscripts.

With the introduction of the Greasemonkey 4 WebExtension, the classic GM_* "synchronous" API is replaced with a new GM.* "asynchronous API". The features of the new API in Greasemonkey 4 (GM4) are very similar to the classic API, but all the functions are asynchronous. This means you might need to (learn asynchronous programming and) do some refactoring of your existing userscripts to make use of the new API in GM4.

As an alternative to refactoring your code, GMC offers a "synchronous subset" of the features found in the GM APIs which works with scripts running in both the new GM4, the older versions of Greasemonkey, and in other userscript managers like Tampermonkey and Violentmonkey. Where APIs are supported in GMC, no refactoring of your script is needed; Simply add the needed @require declaration (and eventually some @grant declarations) to your userscript, and replace the use of GM_* methods with equivalent GMC.* methods.

Currently implemented methods and properties in GMCommonAPI.js are:

• GMC.info
• GMC.registerMenuCommand(caption, commandFunc, accessKey)
• GMC.registerMenuCommand(caption, commandFunc, options)
• GMC.getResourceURL(resourceName)
• GMC.getResourceUrl(resourceName)
• GMC.xmlHttpRequest(details)
• GMC.xmlhttpRequest(details)
• GMC.setValue(name, value) - See compatibility comment in source code!
• GMC.getValue(name, defvalue)
• GMC.deleteValue(name)
• GMC.listValues()
• GMC.setLocalStorageValue(name, value)
• GMC.getLocalStorageValue(name, defvalue)
• GMC.deleteLocalStorageValue(name)
• GMC.listLocalStorageValues()
• GMC.setSessionStorageValue(name, value)
• GMC.getSessionStorageValue(name, defvalue)
• GMC.deleteSessionStorageValue(name)
• GMC.listSessionStorageValues()
• GMC.log(message)
• GMC.setClipboard(text)
• GMC.addStyle(style)
• GMC.openInTab(url)

You can add GMC to your userscript by adding (some of the) following declarations to the Meta data block:

// @grant   GM_registerMenuCommand
// @grant   GM.getResourceUrl
// @grant   GM_getResourceURL
// @grant   GM.xmlHttpRequest
// @grant   GM_xmlhttpRequest
// @grant   GM_setValue
// @grant   GM_getValue
// @grant   GM_deleteValue
// @grant   GM_listValues
// @grant   GM_log
// @grant   GM.setClipboard
// @grant   GM_setClipboard
// @grant   GM_addStyle
// @grant   GM.openInTab
// @grant   GM_openInTab
// @require https://github.com/StigNygaard/GMCommonAPI.js/raw/master/GMCommonAPI.js

Depending on which features you need to use, not all grants are required. Currently you need to check comments in the sourcecode to find the needed @grant declarations for each method (todo: Document it in this README).

You can include GMCommonAPI.js here from GitHub, but you can also find GM Common API on Greasy Fork. To include latest version from Greasy Fork, use:

// @require https://greasyfork.org/scripts/34527/code/GMCommonAPI.js

To freeze the version included from Greasy Fork, use the version parameter as found on Greasy Fork. For example:

// @require https://greasyfork.org/scripts/34527/code/GMCommonAPI.js?version=237580

You can also just copy the complete (or the needed parts of the) javascript code into your userscript if you prefer to do it so. I consider the code Public Domain.

Notice, if you are ready to use an asynchronous API in your userscript (including the refactoring probably needed), you can use a library to wrap the classic API into the new asynchronous API. This will potentially give you a larger cross-compatible API than using GMC. In general, adapting your userscripts to use an asynchronous cross-compatible API with something like gm4-polyfill.js is the recommended way to go forward for optimal performance and "API completeness". GMC might just be a bit easier and faster to use for some of us - or can be used as a temporary quick-fix before refactoring for use of asynchronous API.