JavaScript Source Map Explorer
Upload or paste a JavaScript source map (.map) and instantly see which source files contribute the most bytes to your bundle.
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What is JavaScript Source Map Explorer?
The JavaScript Source Map Explorer analyzes source map version 3 files entirely in your browser. It decodes the base64 VLQ mappings string without any external library, then reports which original files contribute the most bytes via sourcesContent, how many segments point at each source, the most-used names, and overall mapping statistics. Because everything runs locally, you can safely inspect proprietary and internal builds.
Key Benefits
- See exactly which source files dominate your bundle size
- No uploads — safe for proprietary and NDA-protected code
- Works with webpack, Vite, esbuild, Rollup, SWC, and tsc maps
- Constant-memory aggregation keeps multi-megabyte maps responsive
Common Use Cases
- •Auditing which dependencies blew up the main bundle before a release
- •Verifying production builds strip or keep sourcesContent as intended
- •Comparing bundle composition before and after code splitting
- •Investigating a minified stack trace by exploring names and sources
Analyze a JavaScript Source Map
- Load the source map: Paste the .map JSON into the input or press Choose .map file — files up to 50 MB are read locally and never uploaded.
- Read the summary: Six cards show sources, names, segments, generated lines, mappings size, and whether sourcesContent is included, alongside the file, version, and sourceRoot.
- Find the biggest contributors: The Sources by contribution table ranks each original file by content size and share percentage with a bar, so you can see which libraries or directories dominate the bundle.
- Inspect mappings and export: Compare mapped versus unmapped segments and the most-used names, then press Download report to save the full breakdown as a text file.
Key Features
- Dependency-free base64 VLQ decoder for source map version 3 mappings
- Per-source byte sizes and share percentages with visual bars
- Mapping statistics: total, mapped, unmapped, and named segments
- Most-used names list with click-to-copy
- Handles maps up to 50 MB with constant-memory aggregation
- 100% client-side — proprietary code never leaves the browser
Frequently Asked Questions
What does the share percentage in the sources table mean?
When sourcesContent is included it is the exact share of UTF-8 bytes each original file contributes to the bundle; without sourcesContent the tool falls back to estimating shares from mapping segment counts.
What are mapped and unmapped segments?
Each VLQ entry in the mappings field is a segment. Mapped segments point back to an original source line, while unmapped ones (typical for generated or polyfill code) have no origin — a high unmapped count often means generated code inside the bundle.
How big a source map can I analyze?
Files up to 50 MB are accepted, either pasted as text or through the Choose .map file button (.map or .json). Everything is parsed locally in your browser — nothing is uploaded.
What can I do with the names list?
Most-used names are the identifiers referenced most often by mappings — click any chip to copy it for grepping a minified bundle or correlating with profiler output. Download report saves the full breakdown as a text file.