frogcjn/make_VSCode_debug_RN_TS.md

## make_VSCode_debug_RN_TS.md

      
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    If using Typescript to write a ReactNative Project in VSCode, you'll find that VSCode cannot stop at any breakpoint in TypeScript files. You must create breakpoints in JS files.
This post will answer why this problem appears and how to solve that.
Enviroment: VSCode (1.1.1 or 1.2.0) and ReactNative Tool Extension (0.1.4)
Solution Target: Debugger will work on break points in TypeScript files, in a ReactNative Project
Example Project structure:
index.ios.js             // This is the entrance of RN, this entrance leads to `./build/index.ios.js`, which is transformed from TS file
./src/index.ios.tsx      // This is the source code
./build/index.ios.js     // This is generated code
./nodemodules/**/**.js   // This is the JS code from npm

Notes:


tsx files will directly transform to js files, there is no jsx files.

So the jsx in tsconfig.json should be set to react, instead of reserve).
only in this way, you can use TypeScript writing ReactNative projects.
This is becuase tsc in reserve mode will transfer *.tsx to *.jsx file extension, and ReactNative refuse to recognize *.jsx file extension. See facebook/react-native#5233 (comment)


And also it is better to keep an index.ios.js as an pure entrance to your TS code.


Resources


Solution: You can directly clone and run the code to see the solution

VSCode Extension: VSCode ReactNative Tool (revised)
Example ReactNative Project for this issue


Source map Tools

source map visualization


related GitHub repo

[flatten-source-map](flatten-source-map/lib at master · DavidSout…)

mozilla/source-map


VSCode Node Debugger
VSCode Extension: VSCode ReactNative Tool
ReactNative Packager

transformer (using Babel)
bundler


related GitHub issue

facebook/react-native#393
microsoft/vscode-react-native#240
facebook/react-native#7083


The code transform process


TypeScript files --(tsc)--> ES6 files, and source maps.
ES6 files --(ReactNative Packager's transformer (using Babel))--> ES5 files, and source maps
ES5 files --(ReactNative packager's bundler)--> index.ios.bundle & index.ios.map.
VSCode ReactNative Tool copy index.ios.bundle & index.ios.map into .vscode/.react  in your ReactNative project, and let paths in index.ios.map to be relative to it's folder path .vscode/.react.
VSCode Node Debugger running index.ios.bundle  and stop at right places of your break points. This needs helps from index.ios.map.

What's wrong?

###Problem 1:  ReactNative Packager's transformer needs sourceMaps flag in .babelrc
Facebook ReactNative Packager's transformer , using Babel to transform ES6 files to normal ES5 files, plays an important role here.
However, this transformer let the example React Native project decides whether Babel should generate source map. So Babel will not generate source map if not set sourceMaps. Only if you set value with truly value in .babelrc file at your ReactNative project's workspace root, you can get the source map mapping between ES6 files and ES5 files.
###Problem 2: ReactNative Packager's bundler will generate fake source map if no input source map
Wait a moment, there is no souremap between ES6 and ES5 files? But how VSCode still can stop at breakpoints in ES6 files without this source map? I have not set the sourceMapsflag in .babelrc but it works?
Because in step 3, ReactNative packager's bundler will generate fake simple line to line source map mapping between index.ios.bundle and ES5 files generated by Babel. The faked source map looks like:
  {
      "file": "index.ios.bundle",
      "sources": ["../../index.ios.js", "../../src/index.android.tsx", "../../node_modules/react/react.js"],
      "version": 3,
      "names": [],
      "mappings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
      "sourceRoot": ""
  }
It means each line of index.ios.bundle simply mapping to each line of ES5 generated by babel.
(You can use source map visualization to visualize this source map example:)
1:0 -> 1:0 in ../../index.ios.js
2:0 -> 2:0 in ../../index.ios.js
3:0 -> 3:0 in ../../index.ios.js
4:0 -> 4:0 in ../../index.ios.js
5:0 -> 5:0 in ../../index.ios.js
1:0 -> 6:0 in ../../src/index.android.tsx
2:0 -> 7:0 in ../../src/index.android.tsx
3:0 -> 8:0 in ../../src/index.android.tsx
4:0 -> 9:0 in ../../src/index.android.tsx
5:0 -> 10:0 in ../../src/index.android.tsx
6:0 -> 11:0 in ../../src/index.android.tsx
7:0 -> 12:0 in ../../src/index.android.tsx
8:0 -> 13:0 in ../../src/index.android.tsx
9:0 -> 14:0 in ../../src/index.android.tsx
10:0 -> 15:0 in ../../src/index.android.tsx
11:0 -> 16:0 in ../../src/index.android.tsx
...

This source map is lazy  and imprecise, because Babel did not generate source map mapping between ES6 and ES5. Therefore, even if you set breakpoints in ES6 file, not in TS file, the debugger may still stop at wrong places.
OK. So how to solve this problem? We just need a .babelrc file with sourceMaps set to true in your ReactNative project.
After you doing that, you can see a correct source map at http://localhost:8081/index.ios.map?platform=ios&dev=true and.vscode/.react/index.ios.map in the example React Native project. There is a progress, Right?
While,  you cannot set sourceMaps to inline and both, Why? Because ...
###Problem 3: VSCode React Native Tool, cannot deal index.ios.bundle with inline source maps
This means if you set sourceMaps set to inline or both, you'll get in trouble.
VSCode React Native Tool read index.ios.bundle, and find a first match of //# sourceMappingURL=*file url* to get the source map file index.ios.map. This works only with the case that source map are generated in a seperated file.
If there is multiple inline source map with DataURI format //@ sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9..., it will trigger errors.
I have no interest to solve this problem, so let us just set sourceMaps set to true in the example React Native project, ignore other posibilities.


But we still cannot stop any break point in TS files, and recently, we even cannot stop any break point in JS files!!! Why? Are we doing wrong?
No! We are not doing nothing wrong. It is the fault of VSCode Node Debugger(Step 5)!!

Problem 4: VSCode Node Debugger, cannot read source map with sections

If we let ReactNative Packager's transformer (using Babel) generated source map in step 2, then the ReactNative Packager's bundler in step 3 will generated a source map with sections,
source map with sections looks like this:
    {
      "version": 3,
      "file": "/index.ios.bundle?platform=ios&dev=true",
      "sections": [
        {
          "offset":{"line":1442,"column":0},
          "map": {
            "version":3,
            "file":"build/index.ios.js",
            "sourceRoot":"",
            "sources":["../src/index.ios.tsx"],
            "names":[],
            "mappings":";;;;;AAMO,gCAAK,AAAK,AAAM,AAAO,AAC9B,AAAM;;AACC,AACH,AAAU,AACV,AAAI,AACJ,AAAI,AACP,AAAM,AAAc,AAErB,6CAPO,AAAS,AAAC,UAAG,AAAK;;;AASjB,AAAM,AAAC,OACH,MAAC,AAAI,iCAAC,AAAK,MAAE,AAAM,OAAC,AAAU;AAC1B,MAAC,AAAI,iCAAC,AAAK,MAAE,AAAM,OAAC,AAAQ,SACxB,AACJ,AAAO;AACP,MAAC,AAAI,iCAAC,AAAK,MAAE,AAAM,OAAC,AAAa,cAC7B,AACJ,AAAO;AACP,MAAC,AAAI,iCAAC,AAAK,MAAE,AAAM,OAAC,AAAa;AAC7B,AAAuB;AAAC,AAAK;AAC7B,AACJ,AAAO,AACJ,AACV,AACL,gCAAC,AACL,AAAC,6BAjB2C,AAAS,AACjD,AAAM;;;AAkBV,IAAM,AAAM,OAAG,AAAU,wBAAC,AAAM;AAC5B,AAAS;AACL,AAAI,KAAE,AAAC,CADA;AAEP,AAAc,eAAE,AAAQ;AACxB,AAAU,WAAE,AAAQ;AACpB,AAAe,gBAAE,AAAS,AAC7B;;AACD,AAAO,QAAE;AACL,AAAQ,SAAE,AAAE;AACZ,AAAS,UAAE,AAAQ;AACnB,AAAM,OAAE,AAAE,AACb,GAX4B;;AAY7B,AAAY,aAAE;AACV,AAAS,UAAE,AAAQ;AACnB,AAAK,MAAE,AAAS;AAChB,AAAY,aAAE,AAAC,AAClB,AACJ,AAAC"
          }
        },
        {
          "offset":{"line":1478,"column":0},
          "map":...
        }
This kind of source map only supported by Chrome, not suported by VSCode Node Debugger, which is the built-in JS debugger of VSCode.
So we have two choices to solve this problem:

make VSCode Node Debugger understand sectioned source map in step 5
when VSCode React Native Tool copy index.ios.map into the project, which is step 4, flatten the source map with sections to a normal one.

Because it is easy to flatten source map with sections to a normal one using flatten-source-map package, I choose the 2nd approach the solve this problem. (Thanks to @DavidSouther to quickly publish version 0.0.2 of this package.)
So I change the behavior of VSCode React Native Tool, it now will flatten source map with sections to normal source map, so the VSCode Node Debugger can work correctly with source map in the folder .vscode/.react/ of your ReactNative project.
Note: don't worry about source map chain problem


tsc will generated a source map for mapping between TS and ES6


ReactNative Packager's transfer using Babel will generate source map for mapping ES6 and ES5


Do we need to combine this two source maps to make a final source map?
No, we don't need. Since Babel will automatically find #sourceMappingURL= in your ES6 files and read TS-ES6 source mapping  and do the right things. It will generate a correct mapping between the source TS files and generated ES5 files.