Source Map Toolkit

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

Source Map Parsing

The SourcemapToolkit.SourcemapParser.dll provides an API for parsing a souce 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.

Example

Source map string

{
    "version": 3,
    "file": "CommonIntl",
    "mappings": "AACAA,aAAA,CAAc",
    "sources": ["CommonIntl.js"],
    "names": ["CommonStrings", "afrikaans"]
}

Sample source map object

Name	Value	Type
map	{SourcemapToolkit.SourcemapParser.SourceMap}	SourcemapToolkit.SourcemapParser.SourceMap
|--File	"CommonIntl"	string
|--Mappings	"AACAA,aAAA,CAAc"	string
|--Names	Count=2	System.Collections.Generic.List
|--[0]	"CommonStrings"	string
|--[1]	"afrikaans"	string
|--ParsedMappings	Count=3	System.Collections.Generic.List<SourcemapToolkit.SourcemapParser.MappingEntry>
|--[0]	{SourcemapToolkit.SourcemapParser.MappingEntry}	SourcemapToolkit.SourcemapParser.MappingEntry
|--GeneratedSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	0	int
|--ZeroBasedLineNumber	0	int
|--OriginalFileName	"CommonIntl.js"	string
|--OriginalName	"CommonStrings"	string
|--OriginalSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	0	int
|--ZeroBasedLineNumber	1	int
|--[1]	{SourcemapToolkit.SourcemapParser.MappingEntry}	SourcemapToolkit.SourcemapParser.MappingEntry
|--GeneratedSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	13	int
|--ZeroBasedLineNumber	0	int
|--OriginalFileName	"CommonIntl.js"	string
|--OriginalName	null	string
|--OriginalSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	0	int
|--ZeroBasedLineNumber	1	int
|--[2]	{SourcemapToolkit.SourcemapParser.MappingEntry}	SourcemapToolkit.SourcemapParser.MappingEntry
|--GeneratedSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	14	int
|--ZeroBasedLineNumber	0	int
|--OriginalFileName	"CommonIntl.js"	string
|--OriginalName	null	string
|--OriginalSourcePosition	{SourcemapToolkit.SourcemapParser.SourcePosition}	SourcemapToolkit.SourcemapParser.SourcePosition
|--ZeroBasedColumnNumber	14	int
|--ZeroBasedLineNumber	1	int
|--Sources	Count=1	System.Collections.Generic.List
|--[0]	"CommonIntl.js"	string

Usage

The top level API for source map parsing is the SourceMapParser.ParseSourceMap method. The input is a StreamReader 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
SourceMapParser parser = new SourceMapParser();
SourceMap sourceMap;
using (FileStream stream = new FileStream(@"sample.sourcemap", FileMode.Open))
{
    sourceMap = parser.ParseSourceMap(new StreamReader(stream));
}

// Manipulate the source map
...

// Save to source map to file
SourceMapGenerator generator = new SourceMapGenerator();
string serializedMap = generator.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

SourcePosition inOriginal = new SourcePosition { ZeroBasedLineNumber = 34, ZeroBasedColumnNumber = 23 };
SourcePosition inBundled = new SourcePosition { ZeroBasedLineNumber = 23, ZeroBasedColumnNumber = 12 };
SourcePosition inMinified = new SourcePosition { ZeroBasedLineNumber = 3, ZeroBasedColumnNumber = 2 };

MappingEntry originalToBundledEntry = new MappingEntry {
  GeneratedSourcePosition = inBundled,
  OriginalSourcePosition = inOriginal,
  OriginalFileName = "original.js"
};

MappingEntry bundledToMinifiedEntry = new MappingEntry {
  GeneratedSourcePosition = inMinified,
  OriginalSourcePosition = inBundled,
  OriginalFileName = "bundle.js"
};

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

SourceMap 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
SourceMap minifiedToOriginal = minifiedToBundled.ApplySourceMap(bundledToOriginal);

Call Stack Deminification

The SourcemapToolkit.CallstackDeminifier.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.ZeroBasedColumnNumber: 49
    SourcePosition.ZeroBasedLineNumber: 0

Sample Deminified StackFrame entry

    FilePath: "crashcauser.js"
    MethodName: "level1"
    SourcePosition.ZeroBasedColumnNumber: 8
    SourcePosition.ZeroBasedLineNumber: 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 StreamReader 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.

StackTraceDeminifier sourceMapCallstackDeminifier = StackTraceDeminfierFactory.GetStackTraceDeminfier(new SourceMapProvider(), new SourceCodeProvider());
DeminifyStackTraceResult deminifyStackTraceResult = sourceMapCallstackDeminifier.DeminifyStackTrace(callstack);
string 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 occured 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 soure 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 implmentation 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 Json.NET which is licensed under the MIT License.

The call stack deminifier and test app both use Ajax Min which is licensed under the Apache License, Version 2.0.

The unit tests for this library leverage the functionality provided by Rhino Mocks. Rhino Mocks is Open Source and released under the BSD license.

License

Copyright (c) Microsoft Corporation. All rights reserved.

Licensed under the MIT License.