Fix SyntaxError: Illegal return statement

[gitim]

Faced with SyntaxError in this package, because some package in my project depend from this package.

[ljharb]

This is not a syntax error whatsoever; this is a node module and top level return is allowed in it.

What bundler are you using that breaks on valid node modules?

[Jessidhia]

Babel doesn't allow top level return by default; if you use babel-loader to process your dependencies (as recommended by babel(!) for @babel/preset-env) you're going to get this error.

> require('@babel/parser').parse(require('fs').readFileSync(require.resolve('is-map'), 'utf8'))
Thrown:
[SyntaxError: 'return' outside of function (12:1)
] {
  pos: 315,
  loc: Position { line: 12, column: 1 }
}

Top level return is a specific quirk of commonjs modules because of the function wrapper, not actually something that is allowed by script or module (or unambiguous) parse goals, so that on its own doesn't enable parsing it.

This is an example of the whole error stack when building storybook:

ERR! => Failed to build the preview
ERR! ../node_modules/is-map/index.js
ERR! Module build failed (from ../node_modules/thread-loader/dist/cjs.js):
ERR! Thread Loader (Worker 0)
ERR! /builds/afQUGiFh/0/node_modules/is-map/index.js: 'return' outside of function (12:1)
ERR! 
ERR!   10 | 		return false;
ERR!   11 | 	};
ERR! > 12 | 	return;
ERR!      | 	^
ERR!   13 | }
ERR!   14 | 
ERR!   15 | var $mapHas = $Map ? Map.prototype.has : null;
ERR!     at PoolWorker.fromErrorObj (/builds/afQUGiFh/0/node_modules/thread-loader/dist/WorkerPool.js:262:12)
ERR!     at /builds/afQUGiFh/0/node_modules/thread-loader/dist/WorkerPool.js:204:29
ERR!     at mapSeries (/builds/afQUGiFh/0/node_modules/neo-async/async.js:3625:14)
ERR!     at PoolWorker.onWorkerMessage (/builds/afQUGiFh/0/node_modules/thread-loader/dist/WorkerPool.js:170:35)
ERR!     at /builds/afQUGiFh/0/node_modules/thread-loader/dist/WorkerPool.js:152:14
ERR!     at Socket.onChunk (/builds/afQUGiFh/0/node_modules/thread-loader/dist/readBuffer.js:36:9)
ERR!     at Socket.emit (events.js:210:5)
ERR!     at Socket.Readable.read (_stream_readable.js:546:10)
ERR!     at Object.raise (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:6983:17)
ERR!     at Object.parseReturnStatement (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11027:12)
ERR!     at Object.parseStatementContent (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10714:21)
ERR!     at Object.parseStatement (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10666:17)
ERR!     at Object.parseBlockOrModuleBlockBody (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11242:25)
ERR!     at Object.parseBlockBody (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11229:10)
ERR!     at Object.parseBlock (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11213:10)
ERR!     at Object.parseStatementContent (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10742:21)
ERR!     at Object.parseStatement (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10666:17)
ERR!     at Object.parseIfStatement (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11020:28)
ERR!     at Object.parseStatementContent (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10711:21)
ERR!     at Object.parseStatement (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10666:17)
ERR!     at Object.parseBlockOrModuleBlockBody (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11242:25)
ERR!     at Object.parseBlockBody (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:11229:10)
ERR!     at Object.parseTopLevel (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:10597:10)
ERR!     at Object.parse (/builds/afQUGiFh/0/node_modules/@babel/parser/lib/index.js:12107:10)
ERR!  @ ../node_modules/es-get-iterator/index.js 145:16-33
ERR!  @ ../node_modules/iterate-value/index.js
ERR!  @ ../node_modules/promise.allsettled/implementation.js
ERR!  @ ../node_modules/promise.allsettled/polyfill.js
ERR!  @ ../node_modules/promise.allsettled/shim.js
ERR!  @ ../node_modules/promise.allsettled/auto.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2020.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2019.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2018.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2017.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2016.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es2015.js
ERR!  @ ../node_modules/airbnb-js-shims/target/es5.js
ERR!  @ ../node_modules/airbnb-js-shims/index.js
ERR!  @ ../node_modules/@storybook/core/dist/server/common/polyfills.js
ERR!  @ multi ../node_modules/@storybook/core/dist/server/common/polyfills.js ../node_modules/@storybook/core/dist/server/preview/globals.js ./.storybook/config.js

[ljharb]

It's highly unsafe to transpile code you didn't author; is this perhaps because you're trying to process third party code as if it's an ESM Module, instead of a CJS Script? If so, that will break things as well by forcing strict mode.

It's certainly something only allowed because of CJS's function wrapper, but it has always had the function wrapper and that's what makes it valid.