# 20 Mistakes That Quietly Destroy JavaScript/TypeScript Codebases (Part 2)

DevFeed: [20 Mistakes That Quietly Destroy JavaScript/TypeScript Codebases (Part 2)](<https://devfeed.tech/articles/20-mistakes-that-quietly-destroy-javascript-typescript-codebases-part-2-18103.md>)

Original publisher: [Read original article](<https://thetshaped.dev/p/20-mistakes-that-quietly-destroy-javascript-typescript-codebases-common-code-smell-patterns-async-performance-testing>)

Author: The T-Shaped Dev

Published: 2026-05-09T02:19:51Z

Content type: tutorial

Language: en

Sources: [The T-Shaped Dev](<https://devfeed.tech/sources/the-t-shaped-dev.md>)

Topics: [JavaScript](<https://devfeed.tech/topics/javascript.md>), [TypeScript](<https://devfeed.tech/topics/typescript.md>), [Code](<https://devfeed.tech/topics/code.md>), [Node.js](<https://devfeed.tech/topics/node-js.md>), [Testing](<https://devfeed.tech/topics/testing.md>)

Tags: [async](<https://devfeed.tech/tags/async.md>), [code](<https://devfeed.tech/tags/code.md>), [javascript](<https://devfeed.tech/tags/javascript.md>), [memory-leaks](<https://devfeed.tech/tags/memory-leaks.md>), [mistakes](<https://devfeed.tech/tags/mistakes.md>), [node-js](<https://devfeed.tech/tags/node-js.md>), [patterns](<https://devfeed.tech/tags/patterns.md>), [test](<https://devfeed.tech/tags/test.md>), [typescript](<https://devfeed.tech/tags/typescript.md>)

## AI overview

Part 2 of a JavaScript and TypeScript code-quality series presents common runtime and code-quality mistakes, including mutating function parameters, memory leaks from uncleared listeners, timers and subscriptions, and async cancellation issues. It includes fixes and testing or debugging guidance.

## Source excerpt

Common JS/TS patterns that feel fine until they don't. 11 mistakes, before/after code for each. (11 min)