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jsPsych is a JavaScript library for running behavioral experiments in a web browser. The library provides a flexible framework for building a wide range of laboratory-like experiments that can be run online.

To use jsPsych, you provide a description of the experiment in the form of a timeline. jsPsych handles things like determining which trial to run next, storing data, and randomization. jsPsych uses plugins to define what to do at each point on the timeline. Plugins are ready-made templates for simple experimental tasks like displaying instructions or displaying a stimulus and collecting a keyboard response. Plugins are very flexible to support a wide variety of experiments. It is easy to create your own plugin if you have experience with JavaScript programming.

The page on timelines is a good place to start learning about jsPsych. From there, you might want to complete the Hello World! tutorial and the reaction time experiment tutorial.