loadometer

See where your app spends time loading modules, as a flame graph.

loadometer times every module your app loads — require() and import, JavaScript and TypeScript — and writes the result as folded stacks (import;chain milliseconds), the format flame-graph tools understand.

Install

npm install --save-dev loadometer

Usage

Run your app with loadometer as a preload — no code changes:

node --import loadometer/register app.js      # Node
bun  --preload loadometer/register app.js     # Bun

This is the recommended way everywhere. The preload hooks the module loader and times each module's load and evaluation, covering require() and import, in both JavaScript and TypeScript.

Run it: print to the console

By default the folded stacks are printed to stdout:

node --import loadometer/register app.js

./app.js 42
./app.js;express 31
./app.js;express;body-parser 8

Each line is an import chain and how many milliseconds that load took.

Run it: write to a file

Set LOADOMETER_OUT_FILE to write the folded stacks to a file instead:

LOADOMETER_OUT_FILE=imports.folded node --import loadometer/register app.js

Visualize it on the web

Open speedscope.app and drag your imports.folded file onto the page — it renders an interactive flame graph, no install required.

Prefer an SVG? Any folded-stack renderer works:

npx inferno imports.folded > imports.svg
# or Brendan Gregg's script:
flamegraph.pl imports.folded > imports.svg

How it works

loadometer measures the wall-clock time each module takes to load, then writes it as folded stacks. It uses two mechanisms, depending on how a module is loaded:

CommonJS (require). It wraps Module.prototype.require and measures with performance.now() around the original call. Because require() synchronously reads, compiles, and executes the module — and everything that module requires in turn — the time is inclusive: a module's number includes its children.

ESM (import), via the preload. Native imports never call require, and a module's body runs after its dependencies are loaded, so there's no single call to wrap. Instead the loader hooks do two things per module:

1. Time the load step — reading the file and transpiling it (e.g. stripping TypeScript types).

2. Rewrite the module's source to insert timestamps around its body, timing its evaluation (the top-level code actually running). The resolve step records each module's importer, which is how the a;b;c import chain is rebuilt.

   The reported number is load + evaluation for that module.

What a line means. Output is one line per module in folded-stack form:

express;body-parser;bytes 3

reads as "loading bytes, imported by body-parser, imported by express, took 3 ms." Flame-graph tools size each box by summing its children, turning these lines into the familiar width-is-time picture.

It's wall-clock time, not CPU time, so it includes disk I/O and any waiting (such as top-level await). A module that's already cached loads in ~0 ms — the work only happens the first time it's pulled in.

TypeScript

Works out of the box. The preload instruments TypeScript source directly, so it covers Node's built-in type stripping, tsx / ts-node, and Bun's native TS — no separate build step.

Coverage, and the require() workaround

| Runtime | --import / --preload …/register (recommended) | require('loadometer') | |---------|:-------------------------------------------------:|:-----------------------:| | Node | CommonJS + ESM | CommonJS | | Bun | ESM | CommonJS |

The preload is the way to go everywhere — with one exception: on Bun, require()'d CommonJS dependencies bypass Bun's loader plugin, so the preload sees only ESM there.

For a CommonJS app on Bun, use the in-file import as a workaround — drop this at the very top of your entry, before anything else (Bun routes require through Module.prototype.require, so this captures the loads below it):

require('loadometer'); // must be first
require('./app');

Examples

Runnable examples live in examples/ — quick scripts, self-contained repos for every Node / Bun × JS / TS × CommonJS / ESM combination, and a long-running demo server. Run any of them with npm:

npm run example:1            # console output
npm run example:2            # writes imports.folded

npm run example:node-js-esm  # Node + JS + native ESM (preload)
npm run example:bun-ts-esm   # Bun  + TS + native ESM (preload)
npm run example:node-ts-cjs  # Node + TS + CommonJS
# …every runtime × language × module system — 8 in total

npm run example:server       # long-running server: hit /load?pkg=…, then Ctrl+C
npm run examples             # run the whole matrix at once

See examples/README.md for the full matrix. (Bun scripts need bun on your PATH.)

Notes

• What the numbers mean. The preload times each module's load + its own evaluation; the require('loadometer') workaround times each load inclusively (the module and everything it pulls in). Both render fine as a flame graph.