使用Visual Studio Code调试“必需”的Node.js依赖项

Question

I want to use VSC to develop some extensions to an existing project. In particular UglifyJS3.

So I create a test script, in which I call into the module to test and debug my changes. The code there is simply:

var UglifyJS = require("../tools/node");
var result = UglifyJS.minify(code, ...)

This works while debugging/single-stepping. However VSC already fails to resolve the minify function which is exported by UglifyJS.
I am also not able to simply set breakpoints in the UglifyJS files and have them triggered. VSC shows an error on startup about not being able to resolve the breakpoint.

Looking at ../tools/node.js I see code like:

var UglifyJS = exports;
require.resolve("../lib/utils.js")
...
require.resolve("../lib/minify.js")
require.resolve("./exports.js")

with exports.js containing exports["minify"] = minify; .

Is there anything I have missed? How can I make debugging (and optionally IntelliSense) work?

Answer 1

I found the reason:

First clue was that the code showed up in the debugger as <eval> and in a VM<some number> tab. So somehow the code was not actually read from file but dynamically evaluated.

What happens is (to a reduced example):

var fn = require.resolve("<file>"); // Resolve the filename
var code = fs.readFileSync(fn, "utf8");; // Read the file contents
new Function("exports", code)(exports); // Create a new function containing the files code and execute it

The last bit is a bit more cryptic in code and the new Function was new to me (no pun intended). It essentially boils down to an optimized eval with strict-mode compatibility: https://whereswalden.com/2011/01/10/new-es5-strict-mode-support-new-vars-created-by-strict-mode-eval-code-are-local-to-that-code-only/

Hence: The code executed is actually eval'd code and not the files itself.

Solution: Either live with it or change it upstream to "clean" modules.