The deno bench command runs benchmarks to measure and compare the performance of your code.
Usage
deno bench [OPTIONS] [FILES]...
Basic Examples
# Run all benchmarks
deno bench
# Run specific benchmark file
deno bench bench.ts
# Run benchmarks matching a pattern
deno bench --filter "sort"
# Output results as JSON
deno bench --json
Benchmark Discovery
Deno automatically discovers benchmark files with these patterns:
- Files ending with
_bench.ts, _bench.tsx, _bench.js, _bench.jsx
- Files ending with
.bench.ts, .bench.tsx, .bench.js, .bench.jsx
- Files named
bench.ts, bench.tsx, bench.js, bench.jsx
Filtering Options
Run benchmarks with names matching this pattern# String match
deno bench --filter "sorting"
# Regex pattern
deno bench --filter "/^array/"
Ignore files matching this patterndeno bench --ignore=integration/
Output Options
Output benchmark results as JSONdeno bench --json > results.json
Only type-check benchmarks, don’t run them Watch Mode
Watch for file changes and rerun benchmarks# Watch mode
deno bench --watch
# Watch specific paths
deno bench --watch=src/,benches/
Don’t clear the screen on watch mode restart
Permission Flags
All standard Deno permission flags are available:
deno bench --allow-read --allow-net bench.ts
Writing Benchmarks
Basic Benchmark
Deno.bench("string concatenation", () => {
let str = "";
for (let i = 0; i < 1000; i++) {
str += "a";
}
});
Async Benchmark
Deno.bench("async fetch", async () => {
const response = await fetch("https://example.com");
await response.text();
});
Benchmark with Setup and Teardown
Deno.bench({
name: "array operations",
fn: () => {
const arr = Array.from({ length: 1000 }, (_, i) => i);
arr.sort((a, b) => b - a);
},
});
Baseline Comparison
Deno.bench({
name: "string concat - baseline",
baseline: true,
fn: () => {
let str = "";
for (let i = 0; i < 1000; i++) {
str += "a";
}
},
});
Deno.bench({
name: "string concat - array join",
fn: () => {
const arr = [];
for (let i = 0; i < 1000; i++) {
arr.push("a");
}
arr.join("");
},
});
Grouped Benchmarks
Deno.bench({
name: "sort - quicksort",
group: "sorting",
fn: () => {
const arr = Array.from({ length: 1000 }, () => Math.random());
quickSort(arr);
},
});
Deno.bench({
name: "sort - mergesort",
group: "sorting",
fn: () => {
const arr = Array.from({ length: 1000 }, () => Math.random());
mergeSort(arr);
},
});
Warmup Benchmarks
Deno.bench({
name: "computation",
warmup: true,
fn: () => {
// Warmup iteration
},
});
Ignoring Benchmarks
Deno.bench({
name: "expensive operation",
ignore: true,
fn: () => {
// Benchmark code
},
});
Examples
Comparing String Operations
// string_bench.ts
Deno.bench({
name: "concat with +",
baseline: true,
fn: () => {
let result = "";
for (let i = 0; i < 10000; i++) {
result += "test";
}
},
});
Deno.bench("concat with array join", () => {
const parts = [];
for (let i = 0; i < 10000; i++) {
parts.push("test");
}
parts.join("");
});
Deno.bench("template literals", () => {
let result = "";
for (let i = 0; i < 10000; i++) {
result = `${result}test`;
}
});
deno bench string_bench.ts
Benchmarking with JSON Output
# Run benchmarks and save results
deno bench --json bench.ts > results.json
# Process results with jq
deno bench --json bench.ts | jq '.results[] | {name, avg}'