Younghoon Kim Blog

Mar 27, 202610 min read

Why should good tests make failures believable?

When tests in React Router or TanStack Query fail, I usually inspect my code first. When tests I wrote fail, I sometimes end up re-checking the test itself as well. That difference made me rethink the boundary between unit, integration, browser, and e2e tests.

testingvitest+1

Mar 01, 20266 min read

useRef holds values not needed for rendering, so why should you not read it during render?

Two sentences in the React docs looked contradictory at first. useRef stores values not needed for rendering, yet React says not to read ref.current during render. Tracing the source code shows these are about two different dimensions of rendering: trigger vs consistency, all the way to tearing.

reactuseRef+1

Feb 21, 20267 min read

Why is Next.js source code so complex? -- The cost of keeping Pages Router

Tracing just next/navigation through the source reveals where the complexity comes from: re-export entry points, context adapters, and .react-server.ts splits.

nextjssource-code

Feb 13, 202616 min read

Minor optimizations found in React source code

Small React source-level optimizations explained: Object.freeze in DEV, Hidden Class effects, branch ordering, and Smi/Double choices.

react

Feb 10, 20268 min read

ESM is asynchronous, so how is static analysis possible?

ESM can be asynchronous and still statically analyzable. This post explains why through the Construction, Instantiation, and Evaluation phases.

javascriptesm+1

Feb 01, 20265 min read

Why Map and Set are known to have better performance than Array

Why Map and Set often outperform Array: hash-based lookup complexity, insertion behavior, and traversal trade-offs including cache locality.

javascriptdata-structure+1

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Why should good tests make failures believable?

useRef holds values not needed for rendering, so why should you not read it during render?

Why is Next.js source code so complex? -- The cost of keeping Pages Router

Archive

Why should good tests make failures believable?

useRef holds values not needed for rendering, so why should you not read it during render?

Why is Next.js source code so complex? -- The cost of keeping Pages Router

Minor optimizations found in React source code

ESM is asynchronous, so how is static analysis possible?

Why Map and Set are known to have better performance than Array

Why should good tests make failures believable?

useRef holds values not needed for rendering, so why should you not read it during render?

Why is Next.js source code so complex? -- The cost of keeping Pages Router

Why should good tests make failures believable?

useRef holds values not needed for rendering, so why should you not read it during render?

Why is Next.js source code so complex? -- The cost of keeping Pages Router

Minor optimizations found in React source code

ESM is asynchronous, so how is static analysis possible?

Why Map and Set are known to have better performance than Array