CODEWITHSUNDEEP
javascriptbabeljsabstract-syntax-tree
Dan O
Dan O

Reputation: 6090

How do I get @babel/parser to recognize 'undefined' as a special token?

I'm working on a project which involves examining the AST provided by @babel/parser, and in a certain (not super-rare) case it's not behaving as I expected. This line of Javascript:

const var1 = undefined;

when processed by this command:

babelParser.parse(data, {
    plugins: [ `jsx`, `classProperties` ],
    sourceType: `unambiguous`,
});

gets transformed into this subtree:

{
  "type": "VariableDeclarator", // expected
  "start": 240,
  "end": 256,
  "loc": {
    "start": {
      "line": 17,
      "column": 4
    },
    "end": {
      "line": 17,
      "column": 20
    }
  },
  "id": {
    "type": "Identifier", // also expected
    "start": 240,
    "end": 244,
    "loc": {
      "start": {
        "line": 17,
        "column": 4
      },
      "end": {
        "line": 17,
        "column": 8
      },
      "identifierName": "var1"
    },
    "name": "var1"
  },
  "init": {
    "type": "Identifier", // dang, really?
    "start": 247,
    "end": 256,
    "loc": {
      "start": {
        "line": 17,
        "column": 11
      },
      "end": {
        "line": 17,
        "column": 20
      },
      "identifierName": "undefined"
    },
    "name": "undefined"
  }
}

Why does the babel parser treat undefined as a variable instead of "UndefinedLiteral"? (I mean, besides the fact that "UndefinedLiteral" doesn't seem to be a thing according to the AST spec)

Is there a way to change the type of this initial-value node? or will I have to add a special case to my code to look for Identifiers with a value of "undefined"?

Upvotes: 2

Views: 529

Answers (1)

loganfsmyth
loganfsmyth

Reputation: 161517

You can't because undefined is not a special token in the JS grammar. It is perfectly valid to do

var undefined = 4;
console.log(undefined); // logs 4

so you'll need to check for an Identifier called undefined, and if you want to be careful also analyse the scope of the variable to see if it is actually the global undefined binding, or a local one that could have some other value.

Upvotes: 2

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