This repo demonstrates some edge cases that could make Babel compiled code fail.

The src folder contains tests that could be directly excuted with npm run test:src, and all of them pass (with Node.js 10).

npm run build compile the src tests to lib with Babel. Use npm run test:lib to run the compiled tests.

Depending on the options used for Babel (@babel/preset-env), some compiled tests would fail:

• {loose: false, spec: false} (default) 3/10 fail
• {loose: false, spec: true} 2/10 fail
• {loose: true, spec: false} 10/10 fail

Some explanation of the failures

classes

A class must be used as constructor. The classCallCheck method of Babel has false negatives.

https://babeljs.io/docs/en/next/babel-plugin-transform-classes.html

shorthand-properties

Object shorthand methods should not have prototype and cannot be used as constructors.

babel/babel#7115

arrow-functions, spec:false (default)

Arrow functions should not have prototype and cannot be used as constructors.

https://babeljs.io/docs/en/next/babel-plugin-transform-arrow-functions.html#spec

object-rest-spread, loose:true

Should "define" properties instead of using "assign".

https://babeljs.io/docs/en/next/babel-plugin-proposal-object-rest-spread.html#loose

computed-properties, loose:true

Should "define" properties instead of using "assign".

https://babeljs.io/docs/en/next/babel-plugin-transform-computed-properties.html#loose

destructuring, loose:true

Loose destructuring assumes what you want to destructure is an array, and fails for some other types.

https://babeljs.io/docs/en/next/babel-plugin-transform-destructuring.html#loose

parameters, loose:true

Loose parameters with default values fail to keep the function arity for performance trade-off.

https://babeljs.io/docs/en/next/babel-plugin-transform-parameters.html#loose

for-of, loose:true

Loose for-of use a "fast path" for all arrays to achieve better performance, which would fail if the array has its own iterator defined.

https://babeljs.io/docs/en/next/babel-plugin-transform-for-of.html#loose

template-literals, loose:true

Loose template literals use the + operator instead of string.concat, which doesn't handle some special objects well.

https://babeljs.io/docs/en/next/babel-plugin-transform-template-literals.html#loose

spread, loose:true

Loose spread assumes all iterables are arrays, which you may not agree.

https://babeljs.io/docs/en/next/babel-plugin-transform-spread.html#loose

TL;DR

• Babel is not perfect
• Use default options of @babel/preset-env unless you know what you are doing
• Do not write weird code