--- title: Custom Type Guards in Typescript | daily.dev description: Custom type guards in TypeScript allow developers to define functions that return type predicates, enabling precise type narrowing at runtime. A type predicate... canonical: https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Custom Type Guards in Typescript | daily.dev og:description: Custom type guards in TypeScript allow developers to define functions that return type predicates, enabling precise type narrowing at runtime. A type predicate... og:url: https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp og:image: https://api.daily.dev/og/posts/v0MJCXyrp.png og:image:alt: Custom Type Guards in 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. # Custom Type Guards in Typescript **[Playful Programming](https://daily.dev/sources/playfulprogramming)** · [@mainawycliffe](https://daily.dev/mainawycliffe) · 3 min read · 0 upvotes · 0 comments ## Summary Custom type guards in TypeScript allow developers to define functions that return type predicates, enabling precise type narrowing at runtime. A type predicate uses the syntax `parameterName is Type` as a function return type. The post walks through building an `isRectangle` guard using JavaScript's `in` operator to check for specific properties, then demonstrates how it narrows union types inside control flow blocks, eliminating type errors when accessing shape-specific properties. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: --- Tags: [#javascript](https://daily.dev/tags/javascript), [#typescript](https://daily.dev/tags/typescript) [View this post on daily.dev](https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp) ```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":"Custom Type Guards in Typescript","url":"https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp"},"datePublished":"2026-03-16T21:47:43.192Z","dateModified":"2026-03-17T18:00:28.983Z","description":"Custom type guards in TypeScript allow developers to define functions that return type predicates, enabling precise type narrowing at runtime. A type predicate...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/acb03d5b0a5ce484afd41a10563fc998?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/acb03d5b0a5ce484afd41a10563fc998?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Playful Programming","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":"Person","name":"Maina Wycliffe","url":"https://daily.dev/mainawycliffe","image":"https://lh3.googleusercontent.com/a-/AOh14GixkCfcFRGynHA6Q1do3acrQ2SJInlspCdn2KXZWw=s100","description":"Web Developer and Google Developer Expert","interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"EndorseAction"},"userInteractionCount":50}},"commentCount":0,"discussionUrl":"https://daily.dev/posts/custom-type-guards-in-typescript-v0mjcxyrp","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"javascript,typescript","timeRequired":"PT3M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Playful Programming","item":"https://daily.dev/sources/playfulprogramming"},{"@type":"ListItem","position":3,"name":"Custom Type Guards in Typescript"}]} ```