Binding references count isn't always accurate

[tbranyen]

Bug Report

Current Behavior

Using path.scope.getBinding(someIdentifier) doesn't always return an accurate count of someIdentifier usage.

Input Code

• REPL or Repo link if applicable:

const core = require('@babel/core');
const traverse = require('@babel/traverse').default;

const code = `
  function test() {
  }

  export default test();
`;

const { ast, map } = core.transformSync(code, {
  sourceType: 'module',
  ast: true,
});

traverse(ast, {
  Program(path) {
    // Why does this equal 1?
    console.log(path.scope.getBinding('test').references);
  }
});

Expected behavior/code

I am expecting to see the number 2, so that it tracks the declaration and identifier usage in the call.

Environment

• Babel version(s): 7.0.0-rc.1
• Node/npm version: 10
• OS: Linux

Possible Solution

Track all identifier usage across calls.

[babel-bot]

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[nicolo-ribaudo]

I wouldn't expected it to be 2, since test() is a reference to the declaration, but function test() {} isn't referencing anything.

[tbranyen]

@nicolo-ribaudo what you would expect the following to equal?

/* ... */

const code = `
  export const c = false;
  console.log(c);
`;

/* ... */

traverse(ast, {
  Program(path) {
    console.log(path.scope.getBinding('c').references);
  }
});

It's two, it's absolutely inconsistent IMO. One is a declaration in this example and the other is a reference, and yet the count is two. You mentioned that function test() {} isn't referencing anything, which is true, but that's because it got hoisted and literally is the reference in the same way as a variable declaration (which obviously gets counted, per my above example).

This is inconsistent IMO.

[loganfsmyth]

That's definitely inconsistent. export const c = false should not show up in the references.

[loganfsmyth]

Removing this whole block fixes that export const c issue and doesn't break any tests 😬

babel/packages/babel-traverse/src/scope/index.js

Line 88 in 595240f

ExportDeclaration: {

[nicolo-ribaudo]

I think that export declarations are counted because their binding is used (references -> 0 is for unused variables).

[loganfsmyth]

Ohh that makes sense. I don't really know that I like that way of approaching it, but that does explain it.

[tbranyen]

The phrasing "unused variables" sort of contradicts the original post then doesn't it? It seems we're being picky about which identifier usage counts, and which doesn't. This isn't really helpful for the purposes of dead code elimination.