From:  Brian Slesinsky <>
Date:  03 Jun 2014 08:00:35 Hong Kong Time

Re: Adding media type for sources to source maps


It makes sense to me.

- Brian

On Mon, Jun 2, 2014 at 4:01 PM, Andy Sterland 

> Hi all,
> I'm looking to get some feedback on a proposal to add a prefixed* property
> to the source map schema to describe the language of a source file. The
> proposal is to cover both files that are compiled from one source language
> and the rarer case of having multiple languages contributing to the
> compiled file that executes. Though not covered in the proposal below it
> could be useful to also include the language of the generated file e.g.
> text/css or application/javascript etc.
> For the case where there is only one source language the value of the
> x_ms_mediaTypes property is an array with only one string defining a media
> type for all the source files. The string itself should be a media type as
> defined by RFC6838. As there really isn't an exhaustive list of media types
> it's really an opaque string between source map producers and consumers but
> ideally producers should document their media type.
> Single source language example:
> {
>   "version": 3,
>   "file": "combined.js",
>   "sources": ["a.ts", "b.ts"],
>   "names": ["method"],
>   "mappings": "AAAA,...",
>   "x_ms_mediaTypes": ["application/x.typescript;version="]
> }
> For the case where there is mixed source content the source map would look
> like:
> {
>   "version": 3,
>   "file": "combined.js",
>   "sources": ["a.ts", "b_old.ts"],
>   "names": ["method"],
>   "mappings": "AAAA,...",
>   "x_ms_mediaTypes": ["application/x.typescript;version=",
> "application/x.typescript;version="],
>   "x_ms_sourceMediaTypes": [0, 1]
> }
> In this case the x_ms_mediaTypes array is a list of all the unique media
> types and an additional property x_ms_sourceMediaTypes is added to map
> source files to media types using the array indices. In the example below
> the source file b_old.ts is at index 1 of the sources array and is at index
> 1 of the x_ms_sourceMediaTypes array which values points to index 1 of the
> x_ms_mediaTypes which has the value of
> "application/x.typescript;version=".
> The main scenario we want to support is removing ambiguity around the
> source language. In the IE F12 developers we load a language service that
> does things such as syntax coloring the text and determining the span of
> statements etc. In F12 there is a bunch of languages services for
> JavaScript, TypeScript, CSS, CoffeeScript, XML, HTML, C# etc. the trouble
> with source maps is that we don't always know 'which' language service to
> load for source mapped files. Today we base the language service on the URI
> and the content-type header which has had mixed success and more than a few
> sites send info that causes F12 to load the wrong language service, which
> can't be worked around. Beyond the language type itself the version of the
> language is virtually impossible to work out but can be important as
> language versions can have syntactic differences which would require a
> different language service.
> We're looking at adding this in both the TypeScript compiler and the F12
> debugger in IE and would love to get feedback from everyone as it seems
> like it would be useful for all.
> Thoughts?
> -Andy
> * With the hope of it becoming part of the standard in the future.
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