--- title: Object-Oriented Design Principles w/ TypeScript | daily.dev description: Command-Query Separation (CQS) is a design principle stating that a method should either be a COMMAND (performs an action, returns void) or a QUERY (returns... canonical: https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Object-Oriented Design Principles w/ TypeScript | daily.dev og:description: Command-Query Separation (CQS) is a design principle stating that a method should either be a COMMAND (performs an action, returns void) or a QUERY (returns... og:url: https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms og:image: https://api.daily.dev/og/posts/J86RBCQms.png og:image:alt: Object-Oriented Design Principles w/ TypeScript 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. # Object-Oriented Design Principles w/ TypeScript **[Khalil Stemmler](https://daily.dev/sources/khalilstemmler)** · 11 min read · 0 upvotes · 0 comments ## Summary Command-Query Separation (CQS) is a design principle stating that a method should either be a COMMAND (performs an action, returns void) or a QUERY (returns data), but never both. Using TypeScript examples, the post illustrates how mixing commands and queries leads to hard-to-debug side effects (HeisenBugs), inconsistent APIs, and unclear code intent. A Job Recommendations API and a Comment Moderation System are used to show both violations and refactored solutions. The post also maps CQS to CRUD, RESTful HTTP, SQL, use-case design, and introduces CQRS as the architectural-level extension of the principle used in Domain-Driven Design. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [CQRS Without the Astronaut Architecture](https://daily.dev/posts/cqrs-without-the-astronaut-architecture-pu7zqnvab) · The T-Shaped Dev · 84 upvotes · 1 comments --- Tags: [#general-programming](https://daily.dev/tags/general-programming), [#typescript](https://daily.dev/tags/typescript), [#domain-driven-design](https://daily.dev/tags/domain-driven-design) [View this post on daily.dev](https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms) ```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":"Object-Oriented Design Principles w/ TypeScript","url":"https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms"},"datePublished":"2026-06-17T11:25:52.737Z","dateModified":"2026-06-17T11:28:48.712Z","description":"Command-Query Separation (CQS) is a design principle stating that a method should either be a COMMAND (performs an action, returns void) or a QUERY (returns...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/b551a2b735f278776ca7efeed950b8f0?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/b551a2b735f278776ca7efeed950b8f0?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Khalil Stemmler","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Khalil Stemmler","logo":"https://media.daily.dev/image/upload/s--q08_7jGC--/f_auto,q_auto/v1781695534/logos/khalilstemmler?_a=BAMAMiWQ0","url":"https://daily.dev/sources/khalilstemmler"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/object-oriented-design-principles-w-typescript-j86rbcqms","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"general-programming,typescript,domain-driven-design","timeRequired":"PT11M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Khalil Stemmler","item":"https://daily.dev/sources/khalilstemmler"},{"@type":"ListItem","position":3,"name":"Object-Oriented Design Principles w/ TypeScript"}]} ```