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.
"sources": ["a.ts", "b.ts"],
"x_ms_sourcesMurmur2Checksum": [714841382, 473447668 ]