Source Map Tools

This is a C# library for working with JavaScript source maps and deminifying JavaScript callstacks.

This is a fork of microsoft/sourcemap-toolkit project that solves following outstanding issues with original project:

• no active development is done anymore for original project, mostly support-only changes for use inside MS teams
• no nuget publishing for recent changes: #64
• lack of support for modern frameworks (.net core): #57
• lack of support for ES6+: #66

Source Map Parsing

The SourcemapTools.dll provides an API for parsing a source map into an object that is easy to work with and an API for serializing source map object back to json string. The source map class has a method GetMappingEntryForGeneratedSourcePosition, which can be used to find a source map mapping entry that likely corresponds to a piece of generated code.

Usage

The top level API for source map parsing is the SourceMapParser.ParseSourceMap method. The input is a Stream that can be used to access the contents of the source map. The top level API for source map serializing is the SourceMapGenerator.SerializeMapping method. The input is a SourceMap that to be serialized and an optional JsonSerializerSettings that can be used to control the json serialization. A sample usage of the library is shown below.

// Parse the source map from file
SourceMap sourceMap;
using (var stream = new FileStream(@"sample.sourcemap", FileMode.Open))
{
    sourceMap = SourceMapParser.ParseSourceMap(stream);
}

// Manipulate the source map
...

// Save to source map to file
string serializedMap = SourceMapGenerator.SerializeMapping(sourceMap);
File.WriteAllText(@"updatedSample.sourcemap", serializedMap);

Chaining source maps

A common use case when dealing with source maps is multiple mapping layers. You can use ApplySourceMap to chain maps together to link back to the source

var inOriginal = new SourcePosition(34, 23);
var inBundled  = new SourcePosition(23, 12);
var inMinified = new SourcePosition(3 , 2 );

var originalToBundledEntry = new MappingEntry(inBundled, inOriginal, null, "original.js");
var bundledToMinifiedEntry = new MappingEntry(inMinified, inBundled, null, "bundle.js");

var bundledToOriginal = new SourceMap()
{
  File = "bundled.js",
  Sources = new List<string> { "original.js" },
  ParsedMappings = new List<MappingEntry> { originalToBundledEntry }
}

var minifiedToBundled = new SourceMap()
{
  File = "bundled.min.js",
  Sources = new List<string> { "bundled.js" },
  ParsedMappings = new List<MappingEntry> { bundledToMinifiedEntry }
}

// will contain mapping for line 3, column 2 in the minified file to line 34, column 23 in the original file
var minifiedToOriginal = minifiedToBundled.ApplySourceMap(bundledToOriginal);

Call Stack Deminification

The SourcemapToolkit.dll allows for the deminification of JavaScript call stacks.

Example

Call stack string

TypeError: Cannot read property 'length' of undefined
    at i (http://localhost:11323/crashcauser.min.js:1:113)
    at t (http://localhost:11323/crashcauser.min.js:1:75)
    at n (http://localhost:11323/crashcauser.min.js:1:50)
    at causeCrash (http://localhost:11323/crashcauser.min.js:1:222)
    at HTMLButtonElement.<anonymous> (http://localhost:11323/crashcauser.min.js:1:326)

Sample Minified StackFrame entry

    FilePath: "http://localhost:11323/crashcauser.min.js"
    MethodName: "i"
    SourcePosition.Column: 49
    SourcePosition.Line: 0

Sample Deminified StackFrame entry

    FilePath: "crashcauser.js"
    MethodName: "level1"
    SourcePosition.Column: 8
    SourcePosition.Line: 5

Usage

The top level API for call stack deminification is the StackTraceDeminifier.DeminifyStackTrace method. For each url that appears in a JavaScript callstack, the library requires the contents of the JavaScript file and corresponding source map in order to determine the original method name and code location. This information is provided by the consumer of the API by implementing the ISourceMapProvider and ISourceCodeProvider interfaces. These interfaces are expected to return a Stream that can be used to access the contents of the requested JavaScript code or corresponding source map. A StackTraceDeminifier can be instantiated using one of the methods on StackTraceDeminfierFactory. A sample usage of the library is shown below.

var sourceMapCallstackDeminifier = StackTraceDeminifierFactory.GetStackTraceDeminfier(new SourceMapProvider(), new SourceCodeProvider());
var deminifyStackTraceResult     = sourceMapCallstackDeminifier.DeminifyStackTrace(callstack, false);
var deminifiedCallstack          = deminifyStackTraceResult.ToString();

The result of DeminifyStackTrace is a DeminifyStackTraceResult, which is an object that contains a list of StackFrameDeminificationResults which contains the parsed minified StackFrame objects in the MinifiedStackFrame property and an enum indicating if any errors occurred when attempting to deminify the StackFrame. The DeminifiedStackFrame property contains the best guess StackFrame object that maps to the MinifiedStackFrame element with the same index. Note that any of the properties on a StackTrace object may be null if no value could be extracted from the input callstack string or source map.

Memory Consumption

Parsed source maps can take up a lot of memory for large JavaScript files. In order to allow for the StackTraceDeminifier to be used on servers with limited memory resources, the StackTraceDeminfierFactory exposes a GetMethodNameOnlyStackTraceDeminfier method that returns a StackTraceDeminifier that does not keep source maps in memory. Since the StackTraceDeminifier returned from this method only reads the source map once, the deminified stack frames will only contain the deminified method name and will not contain the original source location.

Remarks

Browsers return one based line and column numbers, while the source map spec calls for zero based line and column numbers. In order to minimize confusion, line and column numbers are normalized to be zero based throughout the library.

Acknowledgements

The Base64 VLQ decoding code was based on the implementation in the Closure Compiler which is licensed under the Apache License, Version 2.0.

The source map parsing implementation and the relevant comments were based on the Source Maps V3 spec which is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

The source map parser uses System.Text.Json which is licensed under the MIT License.

The call stack deminifier use Esprima .NET which is licensed under the BSD 3-Clause License.

The unit tests for this library leverage the functionality provided by Moq. Moq is Open Source and released under the BSD 3-Clause License.

License

Licensed under the MIT License.