babel/doc/design/compiler-assumptions.md

Compiler Assumptions

Babel and its associated official transforms make some assumptions about your code. These assumptions are only made as they're either impossible to take into consideration or are extremely exotic.

• undefined, NaN and Infinity have not been externally redefined.
• Built-in objects such as Object, Array, String, Number, Boolean etc have not been redefined.
• Standard methods on built-ins have not been redefined in a way that breaks the original contract.