Clean Code in JavaScript

The in operator

The in operator will return true if a property can be found in an object:

'foo' in { foo: 123 }; // => true

The left-side operand will be coerced to its primitive representation, which, if not Symbol, will be coerced to String. Here, we can see how a left-side operand that is Array will be coerced into a comma-separated serialization of its contents (the native and default way that arrays are coerced to primitives, thanks to Array.prototype.toString):

const object = {  'Array,coerced,into,String': 123};['Array', 'coerced', 'into', 'String'] in object; // => true

All seemingly numeric property names in JavaScript are stored as strings, so accessing someArray[0] is equal to someArray["0"], and therefore enquiring as to ...