Suggest an editImprove this articleRefine the answer for “What is Jest?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**Jest** is a library for testing JavaScript and TypeScript code that automatically checks whether a program works correctly: the developer writes tests, and Jest runs them and shows whether the results matched what was expected. **Key point:** Jest shows exactly where the logic failed instead of just "something went wrong," which makes refactoring safer.Shown above the full answer for quick recall.Answer (EN)Image**Jest** is a library for testing JavaScript and TypeScript code. It lets you automatically check whether a program works correctly. The developer writes tests, and Jest runs them and shows whether the results matched what was expected. --- ### **Why Jest is needed** When developing any program, it is important to make sure the functions work correctly. Instead of manual checks (running the code, logging to the console, guessing where the error is) Jest does this automatically. It runs the tests and immediately shows: - what works, - what broke, - exactly where the error is. --- ### **How Jest helps** - **Checks individual functions** (unit tests) - **Finds errors right away**, before the release - **Makes refactoring safer**: if the tests are green, the logic stayed the same - **Speeds up work**, because tests run instantly - **Shows exactly where the logic failed**, instead of just "something went wrong" --- ### **A simple example** There is a function: ```js function sum(a, b) { return a + b; } ``` Test in Jest: ```js test("sum adds 2 and 3 to equal 5", () => { expect(sum(2, 3)).toBe(5); }); ``` What happens: - Jest runs the test - substitutes 2 and 3 - compares the result with 5 - if the function returned 5 → the test is green - if it returned anything else → the test is red and shows the error --- ### **Why Jest is popular** - very simple syntax - built-in functions for assertions - starts quickly - fits perfectly for React, Node.js and frontend - requires no complex configuration --- ### **Summary** **Jest is a tool that automatically checks the correctness of your JavaScript/TypeScript code. It helps you find errors, write reliable functionality and confidently change the project without breaking the existing logic.**For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.