Function argument destructuring and default value generates "syntax" error

This is a:

• Bug Report
• Feature Request
• Question
• Other

Which concerns:

• flow-runtime
• babel-plugin-flow-runtime
• flow-runtime-validators
• flow-runtime-mobx
• flow-config-parser
• The documentation website

---

What is the current behaviour?

const defaultTrx = 'abc'
class Test {
  static async getLastUpdateTimestamp ({ trx }: { trx: any } = { trx: defaultTrx }) { /* do nothing */ }
}

• flow check itself (flow check --strip-root) does not complain about this
• this function works as expected on runtime

However when I enable babel-plugin-flow-runtime and then I start my program I get following error:

Duplicate declaration "trx"

.babelrc.js:

module.exports = {
  presets: [
    '@babel/flow',
    [ '@babel/env', {
      targets: {
        node: 'current'
      }
    }]
  ],
  plugins: [
    '@babel/proposal-class-properties',
    '@babel/proposal-object-rest-spread',
    '@babel/proposal-export-namespace',
    '@babel/proposal-decorators',
    ['root-import', {
      rootPathPrefix: '@',
      rootPathSuffix: 'src'
    }],
    ['flow-runtime', {
      assert: false,
      annotate: false
    }]
  ],
  env: {
    test: {
      plugins: ['istanbul']
    }
  }
}

Without flow-runtime plugin in .babelrc.js it works during runtime (no syntax errors) and also flow does not complain about it during flow check.

---

What is the expected behaviour?

It should not throw error, this is correct syntax and it’s supported by flow.

---

Which package versions are you using?

"flow-runtime": "^0.17.0",
"babel-plugin-flow-runtime": "^0.17.0",
"@babel/preset-flow": "^7.0.0-beta.32"

[s-ol]

I’m having the same issue:

/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:347
      throw this.hub.buildError(id, `Duplicate declaration "${name}"`, TypeError);
      ^

TypeError: /mnt/space/notification_worker/index.handler.sms.js: Duplicate declaration "timestamp"
  107 |                 })
  108 |               */
> 109 |                 .then(({ timestamp, data }: Notification) => {
      |                          ^
  110 |                   console.log(`${timestamp}: handling ${msg.fields.routingKey} event...`);
  111 |                   return data;
  112 |                 })
    at File.buildCodeFrameError (/mnt/space/notification_worker/node_modules/@babel/core/lib/transformation/file/file.js:261:12)
    at Scope.checkBlockScopedCollisions (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:347:22)
    at Scope.registerBinding (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:504:16)
    at Scope.registerDeclaration (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:444:14)
    at Object.BlockScoped (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:189:28)
    at Object.newFn (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/visitors.js:230:17)
    at NodePath._call (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:53:20)
    at NodePath.call (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:36:14)
    at NodePath.visit (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:88:12)
    at TraversalContext.visitQueue (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/context.js:118:16)

if I work around the destructuring I get another one in a much weirder place:

/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:347
      throw this.hub.buildError(id, `Duplicate declaration "${name}"`, TypeError);
      ^

TypeError: /mnt/space/notification_worker/api/collection.js: Duplicate declaration "cache" (This is an error on an internal node. Probably an internal error.)
    at File.buildCodeFrameError (/mnt/space/notification_worker/node_modules/@babel/core/lib/transformation/file/file.js:261:12)
    at Scope.checkBlockScopedCollisions (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:347:22)
    at Scope.registerBinding (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:504:16)
    at Scope.registerDeclaration (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:444:14)
    at Object.BlockScoped (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/scope/index.js:189:28)
    at Object.newFn (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/visitors.js:230:17)
    at NodePath._call (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:53:20)
    at NodePath.call (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:36:14)
    at NodePath.visit (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/path/context.js:88:12)
    at TraversalContext.visitQueue (/mnt/space/notification_worker/node_modules/@babel/core/node_modules/@babel/traverse/lib/context.js:118:16)

the variable ‘cache’ is the one in this helper (verified by renaming, the error log changes with it):

  buildCache() {
    type Extended = Raw & Helpers;
    const cache: { [string]: Extended } = {};

    return (_id: string): Promise<Extended> => {
      if (cache[_id]) return Promise.resolve(cache[_id]);

      return this.findOne({ _id })
        .then(doc => {
          cache[_id] = doc;
          return doc;
        });
    };
  }

personally I don’t see any commonality between these two cases but maybe it can help someone else figure the issue out as it seems to be related as per the error message. Luckily in my project flow-runtime isn’t used extensively so I will be able to remove it for the time being.

[linonetwo]

My God! Finally, thank you!