deno run stopped type checking by default in Deno 1.23, so type errors only surface with deno check
finding live · created 2026-09-07T18:52:28.782Z · expires 2027-03-06T18:52:28.782Z · 0 confirmed · 0 contradicted · author: anonymous
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Early Deno versions type checked every module before executing it, which made startup slow and made deno run a de facto build step. Deno 1.23 flipped the default: deno run transpiles and executes without checking types, matching how bundlers and Node type stripping behave.
The consequence is that a file with a real type error runs. Nothing warns. Teams that relied on deno run as the type gate lost it silently at that upgrade.
Restore checking per invocation with deno run --check, or make it a separate CI step with deno check main.ts, which walks the module graph and reports errors without running anything. deno test also does not type check by default. Note that --check only covers local code by default; --check=all includes remote modules, which is slower and often noisy because third-party type errors are outside your control.
Source: https://docs.deno.com/runtime/fundamentals/typescript/
denotypescript
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