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Reason: Not obvious that this is significantly different between 2.0 and 3.0
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This article or section documents an outdated version of Scratch (version 2.0). For this article in the current version, see Scratch Extension (3.0).
The LEGO WeDo 2.0 extension blocks
The button to add an extension in Scratch 2.0.

Scratch Extensions make it possible to connect Scratch projects with external hardware (such as LEGO WeDo or micro:bit), sources of information on the web (such as Google Translate and Amazon Text to Speech), or blocks allowing for more advanced functionality. They add a collection of command and reporter blocks that can be used to interact with a particular device or data source. When an extension is enabled, its blocks appear in a location with the same name as the extension.

To load an extension, click the icon in the bottom-left hand corner of the screen and select an extension.

The following extensions were in 2.0 and removed in 3.0:

The following allowed users to write their own extensions in 2.0:

JavaScript Extensions

JavaScript could also be used to make extensions for Scratch 2.0. The official documentation for creating this kind of extension for Scratch 2.0 could be found on the ScratchX Github wiki.

The 2.0 extensions available in the offline editor, like the PicoBoard extension, were JavaScript extensions.

HTTP Extensions

Note Note: This mechanism worked only with the offline editor.
The A4S (Arduino for Scratch) experimental HTTP extension.

Scratch 2.0 could not interact with hardware devices directly. Instead, hardware extensions came with a helper app — a separate application that the user needed to install and run on their computer. Scratch communicated with the helper app via HTTP requests, and the helper app talked to the hardware.

A document (last updated: September 2013) had been drafted for Scratch 2.0 HTTP extension developers to describe the extension description file format — the protocol used to communicate between Scratch extension helper apps, and the extension development process. This specification was still preliminary, and was discontinued due to the release of Scratch 3.0.

The Scratch 2.0 extension mechanism was still under development, and the Scratch Team was still figuring out how the extensions will be shared and distributed. However, it was discontinued upon the release of Scratch 3.0.

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