--- title: Typescript - Tips & Tricks - Readonly Array | daily.dev description: A short tip demonstrating how mutable array methods like `sort()` and `reverse()` silently mutate the original array in JavaScript/TypeScript. Using the... canonical: https://daily.dev/posts/typescript---tips-tricks---readonly-array-iqipqj9eb twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Typescript - Tips & Tricks - Readonly Array | daily.dev og:description: A short tip demonstrating how mutable array methods like `sort()` and `reverse()` silently mutate the original array in JavaScript/TypeScript. Using the... og:url: https://daily.dev/posts/typescript---tips-tricks---readonly-array-iqipqj9eb og:image: https://api.daily.dev/og/posts/iQiPqJ9EB.png og:image:alt: Typescript - Tips & Tricks - Readonly Array og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Typescript - Tips & Tricks - Readonly Array **[Playful Programming](https://daily.dev/sources/playfulprogramming)** · [@puppo](https://daily.dev/puppo) · 2 min read · 0 upvotes · 0 comments ## Summary A short tip demonstrating how mutable array methods like `sort()` and `reverse()` silently mutate the original array in JavaScript/TypeScript. Using the `readonly` modifier on array parameters in TypeScript prevents accidental mutation by raising a compile-time error when mutating methods are called, forcing developers to use non-mutating alternatives like `slice()` first. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Bring Immutability and Context to Arrays](https://daily.dev/posts/bring-immutability-and-context-to-arrays-mipldambi) · TypeScript.TV · 13 upvotes · 0 comments - [Experiment: making TypeScript immutable-by-default](https://daily.dev/posts/experiment-making-typescript-immutable-by-default-wsz7yxyng) · Hacker News · 0 upvotes · 0 comments - [Beyond the basics: 21 TypeScript features you might not know about](https://daily.dev/posts/beyond-the-basics-21-typescript-features-you-might-not-know-about-oivxxtrie) · DEV · 27 upvotes · 0 comments --- Tags: [#webdev](https://daily.dev/tags/webdev), [#typescript](https://daily.dev/tags/typescript) [View this post on daily.dev](https://daily.dev/posts/typescript---tips-tricks---readonly-array-iqipqj9eb) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Typescript - Tips & Tricks - Readonly Array","url":"https://daily.dev/posts/typescript---tips-tricks---readonly-array-iqipqj9eb","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/typescript---tips-tricks---readonly-array-iqipqj9eb"},"datePublished":"2026-03-16T21:46:32.099Z","dateModified":"2026-03-17T18:02:30.214Z","description":"A short tip demonstrating how mutable array methods like `sort()` and `reverse()` silently mutate the original array in JavaScript/TypeScript. 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