Bun is a fast, all-in-one toolkit for installing, bundling, running and testing JavaScript & TypeScript. To install: `npm i -g bun`
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ALT Dark slide with the Bun logo and a "new in Bun" badge. Large headline: "Bun.FetchSession". Subtitle: "fetch() with its own connection pool." Below it, a code sample: const api = new Bun.FetchSession({ proxy: "http://proxy:8080", tls: { ca }, keepAlive: { idleTimeout: 30 }, }); await api.fetch(url); new Octokit({ request: { fetch: api.fetch }, }); "api.fetch" is highlighted in both places. Three cards at the bottom read: "Own pool: isolated keep-alive", "Proxy + TLS: set once per session", "Drop-in: works in any SDK".
ALT Table: "Bun v1.4.1 reduces idle memory usage." Resident memory 3 minutes after 60 seconds of sustained load, lower is better. Columns: Bun 1.4.1, change vs 1.4.0, Bun 1.4.0, Bun 1.3.14, Node.js 26.7. Next.js SSR: 142 MB (−36%), 222, 1303, 195. Vite dev: 157 MB (−17%), 189, 303, 155. Express: 54 MB (−17%), 65, 77, 81. Fastify: 57 MB (−16%), 68, 80, 91. Elysia: 44 MB (no change), 46, 69, 96. Hono: 34 MB (no change), 35, 53, 92. Method: 60 s at a fixed request rate (40k req/s Elysia and Hono, 25k Express and Fastify, 5k Vite dev, 3k Next.js SSR), 64 connections, then 180 s idle; RSS sampled every 100 ms on Linux x64.
![Benchmark 1: bun --print 1234
Time (mean ± σ): 3.8 ms ± 0.2 ms [User: 2.8 ms, System: 1.0 ms]
Range (min … max): 3.4 ms … 4.5 ms 795 runs
Benchmark 2: bun-1.4.0 --print 1234
Time (mean ± σ): 4.9 ms ± 0.3 ms [User: 3.2 ms, System: 1.7 ms]
Range (min … max): 4.1 ms … 5.6 ms 544 runs
Benchmark 3: node --print 1234
Time (mean ± σ): 26.9 ms ± 1.8 ms [User: 14.6 ms, System: 11.9 ms]
Range (min … max): 23.7 ms … 30.7 ms 113 runs
Summary
bun --print 1234 ran
1.27 ± 0.11 times faster than bun-1.4.0 --print 1234
7.01 ± 0.58 times faster than node --print 1234](/pic/orig/media%2FHRV2Zjva0AA0taP.jpg)
ALT Benchmark 1: bun --print 1234 Time (mean ± σ): 3.8 ms ± 0.2 ms [User: 2.8 ms, System: 1.0 ms] Range (min … max): 3.4 ms … 4.5 ms 795 runs Benchmark 2: bun-1.4.0 --print 1234 Time (mean ± σ): 4.9 ms ± 0.3 ms [User: 3.2 ms, System: 1.7 ms] Range (min … max): 4.1 ms … 5.6 ms 544 runs Benchmark 3: node --print 1234 Time (mean ± σ): 26.9 ms ± 1.8 ms [User: 14.6 ms, System: 11.9 ms] Range (min … max): 23.7 ms … 30.7 ms 113 runs Summary bun --print 1234 ran 1.27 ± 0.11 times faster than bun-1.4.0 --print 1234 7.01 ± 0.58 times faster than node --print 1234
ALT Table: "Bun v1.4.1 reduces idle memory usage." Resident memory 3 minutes after 60 seconds of sustained load, lower is better. Columns: Bun 1.4.1, change vs 1.4.0, Bun 1.4.0, Bun 1.3.14, Node.js 26.7. Next.js SSR: 142 MB (−36%), 222, 1303, 195. Vite dev: 157 MB (−17%), 189, 303, 155. Express: 54 MB (−17%), 65, 77, 81. Fastify: 57 MB (−16%), 68, 80, 91. Elysia: 44 MB (no change), 46, 69, 96. Hono: 34 MB (no change), 35, 53, 92. Method: 60 s at a fixed request rate (40k req/s Elysia and Hono, 25k Express and Fastify, 5k Vite dev, 3k Next.js SSR), 64 connections, then 180 s idle; RSS sampled every 100 ms on Linux x64.
ALT Table: "Bun v1.4.1 peak memory usage." Peak resident memory (kernel high-water mark) during 60 seconds of sustained load, lower is better. Columns: Bun 1.4.1, change vs 1.4.0, Bun 1.4.0, Bun 1.3.14, Node.js 26.7. Fastify: 94 MB (−27%), 128, 163, 154. Next.js SSR: 265 MB (−11%), 299, 1653, 387. Vite dev: 280 MB (no change), 296, 495, 243. Elysia: 54 MB (no change), 57, 75, 115. Express: 86 MB (no change), 89, 153, 143. Hono: 48 MB (no change), 47, 63, 121. Same method: fixed request rate for 60 s, 64 connections, Linux x64.
ALT Wall clock, min of 3 interleaved runs × min of 5 iterations, taskset to 8 cores, store active (node v26.3.0 on the same box for reference): ns/op node 26 bun main this PR als.run(v, fn) 410 28.8 15.9 3× nested run() 2833 338 57.8 await <value> 36.8 44.1 34.5 await Promise.resolve() 55.9 48.6 35.5 for await (async generator) 162 110 88.3 N × pending.then(f) + Promise.all 435 345 281
ALT Slide titled “Route navigation on 4G”, subtitle “click → lazy route’s chunks fetched, parsed, and rendered by React · lower is better”. Three horizontal bars: Bun 1.4.0, 1104 ms (dark gray); Vite 8, 757 ms (gray); Bun 1.4.1, 681 ms (pink-to-orange gradient, labeled −38%). Table below with columns Bun 1.4.0 / Vite 8 / Bun 1.4.1: Requests per navigation 108 / 108 / 61; LCP, cold first load 736 ms / 560 ms / 576 ms; JS chunks emitted 918 / 908 / 750; Total JS shipped (gzip) 1.75 MB / 1.67 MB / 1.61 MB. Footnote: 200 lazy routes, 800 components, 30 npm deps, React Compiler on in both, headless Chrome, 170 ms RTT / 9 Mbps, cold cache, median of 7 runs × 12 routes; Bun built with bun build –splitting –min-chunk-size=16384 –react-compiler; Vite 8.2 with @vitejs/plugin-react 6 compiler: true.
ALT Tree-shaking through dynamic import()# bun build now removes the exports of a dynamically imported module that your code never reads. main.ts // is.ts exports isNumber, isOdd, and isEven const { isOdd } = await import("./is"); console.log(isOdd(3)); With bun build ./main.ts --splitting --outdir dist, is.ts becomes its own chunk: dist/is-1qza7reb.js function isNumber(n) { return typeof n === "number"; } function isOdd(n) { return isNumber(n) && n % 2 === 1; } function isEven(n) { return !isOdd(n); } export { isEven, isNumber, isOdd };
ALT Smaller bundles for libraries that use export * as# Libraries like zod and Effect group their exports with export * as. When you called z.object(), Bun 1.4.0 kept every export of that group, because it built a namespace object with a getter for each one. Now z.object points straight at the object function, and the exports you don't use are tree-shaken. out.js var User = exports_external.object({ a: exports_external.string() }); var User = object({ a: string2() }); Each program below calls two or three functions from one library, built with bun build --minify: Package Bun 1.4.0 Bun 1.4.1 Change fp-ts 2.16 21.8 KB 3.2 KB −85% zod 4.5 375.3 KB 77.3 KB −79% ox 1.7 15.4 KB 4.7 KB −70% effect 3.22 369.1 KB 163.6 KB −56% @sinclair/typebox 0.34 107.1 KB 53.3 KB −50%