Blockly is a client-side library for the programming language JavaScript for creating block-based visual programming languages (VPLs) and editors. A project of Google, it is free and open-source software released under the Apache License 2.0. It typically runs in a web browser.
Blockly uses visual blocks that help simplify programming, and can generate code in JavaScript, Lua, Dart, Python, or PHP.
Blockly development began in summer 2011.
In October 2025, the Raspberry Pi Foundation announced that from 10 November 2025, the Blockly open source library and assets, and key members of the Blockly team will transition from Google to the Raspberry Pi Foundation.
The default graphical user interface (GUI) of the Blockly editor consists of a toolbox, and a workspace, where a user can drag and drop and rearrange blocks. The workspace also includes, by default. The editor can be modified easily to customize and limit the available editing features and blocks.
Blockly includes a set of visual blocks for common operations, and can be customized by adding more blocks. New blocks require a block definition and a generator. The definition describes the block's appearance (user interface) and the generator describes the block's translation to executable code. Definitions and generators can be written in JavaScript, or using a visual set of blocks, the Block Factory, which allows new blocks to be described using extant visual blocks; the intent is to make creating new blocks easier.
Blockly is used in several notable projects, including: