Bug Report

This related to #8617

• I would like to work on a fix!

Current Behavior
@babel/parser has a different behavior when estree enabled, and when not.

Input Code
When estree disabled code with Duplicated variable declaration throws while parsing:

const {parse} = require('@babel/parser');
const traverse = require('@babel/traverse').default;

const code = `function square(n) {
  const n = 1;
}`;

// crash with an error: Identifier 'n' has already been declared (1:16)
const ast = parse(code);

When estree mode enabled code is parsed, but can't be traversed because of this.hub is undefined:

const {parse} = require('@babel/parser');
const traverse = require('@babel/traverse').default;

const code = `function square(n) {
  const n = 1;
}`;

const ast = parse(code, {
    plugins: [
        'estree',
    ]
});

traverse(ast, {
  enter(path) {
      console.log('n');
  }
});

/*
    throw this.hub.buildError(id, `Duplicate declaration "${name}"`, TypeError);
                     ^

TypeError: Cannot read property 'buildError' of undefined
    at Scope.checkBlockScopedCollisions (/home/coderaiser/node_modules/@babel/traverse/lib/scope/index.js:402:22)
*/

Same result when instead of estree plugin, errorRecovery enabled.

Expected behavior/code
Would be great if in estree mode babel throws while parsing and/or can throw while traversing
about Duplicate declaration.

Environment

• Babel version(s): v7.9.4
• Node/npm version: Node 12(13)/npm 6
• OS: Ubuntu
• Monorepo: no
• How you are using Babel: parse, traverse

Possible Solution

I suggest to use TypeError, because this is what hub actually does, this way:

throw TypeError(`Duplicate declaration "${name}"`);

If of course buildError doesn't used in development purpose to set breakpoint on error building, anyways Chrome Developer Tools debugger already suggest similar feature pause on exceptions (caught and uncaught).