CLASSES AND MODULES
Classes and modules are two mediums of code containers introduced in ECMAScript 6. There is no concept of an ECMAScript predecessor for either of these, and therefore there is no need to support syntactical compatibility with legacy ECMAScript versions. Therefore, TC-39 decided that all code defined inside ES6 classes and modules by default is in strict mode.
For classes, this includes both class declarations and class expressions; the constructor, instance methods, static methods, getters, and setters are all in strict mode. For modules, all code defined inside them will be in strict mode.
