[Bug]: Nodes without loc included in sourcemap

[DylanPiercey]

💻

• Would you like to work on a fix?

How are you using Babel?

Programmatic API (babel.transform, babel.parse)

Input code

Babel seems to be assigning source locations to nodes which do not have a source location at all.
The below example code adds some ast without a sourcemap around an existing node.

In this case the source map for the output || 1 points to the b identifier in the source code.
This is especially problematic for code coverage which will cause this output code to count as a potentially uncovered branch.

import * as t from "@babel/types";
import * as babel from "@babel/core";
const result = await babel.transformAsync(`a = b`, {
  babelrc: false,
  configFile: false,
  sourceMaps: "inline",
  plugins: [
    {
      visitor: {
        AssignmentExpression(assignmentExpression) {
          assignmentExpression
            .get("right")
            .replaceWith(
              t.logicalExpression(
                "||",
                assignmentExpression.get("right").node,
                t.numericLiteral(1)
              )
            );
        },
      },
    },
  ],
});

Outputs the following code:

a = b || 1;
//# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozLCJuYW1lcyI6WyJhIiwiYiJdLCJzb3VyY2VzIjpbInVua25vd24iXSwic291cmNlc0NvbnRlbnQiOlsiYSA9IGIiXSwibWFwcGluZ3MiOiJBQUFBQSxDQUFDLEdBQUdDLENBQUMifQ==

You can view that the sourcemap is incorrect here https://evanw.github.io/source-map-visualization/#MjMyAGEgPSBiIHx8IDE7Ci8vIyBzb3VyY2VNYXBwaW5nVVJMPWRhdGE6YXBwbGljYXRpb24vanNvbjtjaGFyc2V0PXV0Zi04O2Jhc2U2NCxleUoyWlhKemFXOXVJam96TENKdVlXMWxjeUk2V3lKaElpd2lZaUpkTENKemIzVnlZMlZ6SWpwYkluVnVhMjV2ZDI0aVhTd2ljMjkxY21ObGMwTnZiblJsYm5RaU9sc2lZU0E5SUdJaVhTd2liV0Z3Y0dsdVozTWlPaUpCUVVGQlFTeERRVUZETEVkQlFVZERMRU5CUVVNaWZRPT0xMTUAeyJ2ZXJzaW9uIjozLCJuYW1lcyI6WyJhIiwiYiJdLCJzb3VyY2VzIjpbInVua25vd24iXSwic291cmNlc0NvbnRlbnQiOlsiYSA9IGIiXSwibWFwcGluZ3MiOiJBQUFBQSxDQUFDLEdBQUdDLENBQUMifQ==

Configuration file name

No response

Configuration

No response

Current and expected behavior

Currently the output sourcemap contains unnecessary and invalid additional mappings.

I would expect that only mappings related to nodes with a loc get output in the sourcemap.

Environment

System:
OS: macOS 13.5
Binaries:
Node: 18.18.0 - ~/.nvm/versions/node/v18.18.0/bin/node
Yarn: 1.22.19 - /opt/homebrew/bin/yarn
npm: 9.8.1 - ~/.nvm/versions/node/v18.18.0/bin/npm
pnpm: 8.8.0 - ~/pnpm/pnpm
npmPackages:
@babel/core: ^7.23.2 => 7.23.2

Possible solution

The issue appears to be related to this code which is not cleared and applies it's position to future nodes.

Additional context

No response

[babel-bot]

Hey @DylanPiercey! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite.

[SiddiqAM1]

@DylanPiercey How Do you Know the Posibble solution. just curious . new to open source. What are the prerequisite knowledge required to fin solutions to these problems

[nicolo-ribaudo]

@DylanPiercey What would you expect the source map to be like? From what I see in that example, it looks like Babel is mapping four points: the beginning and and of a and of b.

Btw, I recommend using https://sokra.github.io/source-map-visualization instead of https://evanw.github.io/source-map-visualization, as it also lists the decoded mappings: https://sokra.github.io/source-map-visualization/#base64,YSA9IGIgfHwgMTsKLyogYmFzZTY0IHNvdXJjZSBtYXAgcmVtb3ZlZCAqLwo=,eyJ2ZXJzaW9uIjozLCJuYW1lcyI6WyJhIiwiYiJdLCJzb3VyY2VzIjpbInVua25vd24iXSwic291cmNlc0NvbnRlbnQiOlsiYSA9IGIiXSwibWFwcGluZ3MiOiJBQUFBQSxDQUFDLEdBQUdDLENBQUMiLCJmaWxlIjoiZXhhbXBsZS5qcyJ9,YSA9IGI=

[DylanPiercey]

@nicolo-ribaudo the problem is that final mapping of the || 1. That code is not in the source and should not be mapped back anywhere.

My main issue is I've got a Babel transform that outputs something like this, but the || 1 is being marked as an uncovered branch in the test coverage. This ends up being very confusing because this code is not in the source at all. Generally my understanding is that nodes added (not in the source) should not be added to the source map.

[DylanPiercey]

@SiddiqAM1 I scanned through @babel/generator which is what creates the source maps. While debugging through that I could see the reason for the additional mappings was because of a long lived _sourcePosition which I believe should be cleared when we enter a new unmapped node.

[nicolo-ribaudo]

@DylanPiercey Source maps do not map "source ranges", but they map "source points". In this case, we are mapping these points of the output (adding _ between characters just for clarity):

_a_ _=_ _b_ _||_ _1_;_
↓ ↓     ↓ ↓
_a_ _=_ _b_;_

We are not mapping || 1 explicitly to anything.

[DylanPiercey]

@nicolo-ribaudo am I wrong in thinking that the map should be reset or something like that (pardon my lack of understanding) after the b though? It just confuses me that both sourcemap visualizers show the || 1 as mapped (I see that it's a "point" rather than a range as you say) and that this mapping is seemingly also picked up via sourcemap consumers.

Appreciate your time discussing this!

[nicolo-ribaudo]

Source map don't really support "resetting". One possible way is to generate a source map referring to two source files: one is your original file and one is just an empty virtual file. Then, all the generated code could point to the empty file. However, this feels very hacky 😬

We are working on a proper source maps spec, and there is some relevant discussion going on at tc39/source-map#32.

[DylanPiercey]

@nicolo-ribaudo really appreciate the link. In that issue they bring up terser/terser#1106 (comment) as a similar issue. Although from what I understand they fixed that on the terser side.

It looks like they did this here: https://github.com/terser/terser/pull/1106/files#diff-1378f0e2a198059d9d7cf6a4718b869f277986aef3198af9d92af29e1ce2f906R93-R98

Is that something babel could do as well?

[jridgewell]

Source map don't really support "resetting".

Source maps have a "sourceless" segment ([genColumn] segment instead of a "source" segment [genColumn, sourcesIndex, sourceLine, sourceColumn] or "named" segment [genColumn, sourcesIndex, sourceLine, sourceColumn, namesIndex]), which is for exactly this purpose. But, the problem here isn't the || 1 that we're inserting, Babel isn't marking that with source index. It's the AssignmentExpression itself and the parent ExpressionStatement, we're marking the end locations of those two nodes.

Compare:

• https://astexplorer.net/#/gist/628acbcccb939ae5306d94bf7e48830c/0f0821c979b3fccc3987c20e708d9b98e0570f39
• https://astexplorer.net/#/gist/3437c5e22cc1debd5bbf6737868346d9/2e3c37044e27a456833fb0e616b9f1664016e6ed