From:  Andy Sterland <>
Date:  10 Jun 2014 06:43:41 Hong Kong Time

Adding checksums to source maps


One of the features we're adding to the IE F12 developer tools is the ability for developers to load a source map for any file in the debugger without the need for a comment in the file or in a http header. This would enable scenarios where developers have shipped generated JavaScript files to a server and either strip out comments or simply don't want to publish the sourcemap file and/or sources. Basically a mechanism to load symbols :).

This feature would decouple the link between the generated file and the source map which makes it easier to load a source map unrelated to either the source or generated files. In cases where the developer has loaded the wrong source map we'd like to warn them in the debugger that the sources don't match so they can load another source map or carry on regardless. To do this we need to do know that the sources don't all line up which can be worked out by embedding a checksum for both the generated and source files in the source map which the consumer (F12) can them compare. Of course it would be up to the consumer and producers to do what they felt best and these properties would be optional. Right now we're looking at adding them to the TypeScript compiler and F12.

Basically the proposal would add two new properties that would contain the checksums. The proposal below has them as Murmur 2 hashes but this is a key point to get feedback on especially if it should be something like SHA1, SHA2, MD5, CRC32 etc. Overall we choose Murmur v2 because it's fast, widely available, and used in the TypeScript compiler already.

x_ms_fileMurmur2Hash - This property of type number would be the Murmur2 generated hash for the file. This hash should match the entire contents of the file as received by the debugger. Consumers may take liberties with this to account for other build steps for example the hash could be of the entire file excluding any comments at the end of the file*.

x_ms_sourcesMurmur2Hash - This property is an array of numbers each of which is an Murmur2 generated hash for a source file correlated by their indices in sources.

  "version": 3,
  "file": "combined.js",
  "sources": ["a.ts", "b.ts"],
  "names": ["method"],
  "mappings": "AAAA,...",
  "x_ms_fileMurmur2Checksum": 398506827,
  "x_ms_sourcesMurmur2Checksum": [714841382, 473447668 ]



* It's a bit of a complication but our implementation in F12/VS needs to take a checksum of the generated file that excludes trailing comments at the end of the file. We need to do this mainly because some build systems like to add a signature at the end of JavaScript files as part of a signing process that happens after compilation. Thus the debugger that consume a source map and uses this feature will need to be tolerant and compare a checksum that excluded any trailing comments in the file (inc. the sourceMappingURL comment).