# How Jane Street Uses Expect Tests for Fast, Interactive Test Development

DevFeed: [How Jane Street Uses Expect Tests for Fast, Interactive Test Development](<https://devfeed.tech/articles/what-if-writing-tests-was-a-joyful-experience-20214.md>)

Original publisher: [Read original article](<https://blog.janestreet.com/the-joy-of-expect-tests/>)

Author: James Somers

Published: 2023-01-09T00:00:00Z

Content type: article

Language: en

Sources: [Jane Street](<https://devfeed.tech/sources/jane-street.md>)

Topics: [Testing](<https://devfeed.tech/topics/testing.md>), [Jupyter Notebook](<https://devfeed.tech/topics/jupyter-notebook.md>), [Emacs](<https://devfeed.tech/topics/emacs.md>), [Programming](<https://devfeed.tech/topics/programming.md>)

Tags: [emacs](<https://devfeed.tech/tags/emacs.md>), [jest](<https://devfeed.tech/tags/jest.md>), [jupyter-notebook](<https://devfeed.tech/tags/jupyter-notebook.md>), [programming](<https://devfeed.tech/tags/programming.md>), [testing](<https://devfeed.tech/tags/testing.md>), [tests](<https://devfeed.tech/tags/tests.md>)

## AI overview

The article explains Jane Street's expect tests, a testing workflow that fills blank expected-output blocks with program results, shows diffs, and lets developers accept those changes directly in Emacs. It presents the approach as a fast way to explore examples and iteratively improve code.

## Source excerpt

At Jane Street we use a pattern/library called "expect tests" that makes test-writing feel like a REPL session, or like exploratory programming in a Jupyter notebook--with feedback cycles so fast and joyful that it feels almost tactile. Having used them for some time now this is the only way I'd ever want to write tests.