What is the difference between ' and " in JavaScript?

Although not technically a difference in Javascript, its worth noting that single quoted strings are not valid JSON, per se. I think that people automatically assume that since JSON is valid JS, that valid JS strings are also valid JSON, which isn't necessarily true.

E.g., {'key': 'Some "value"'} is not valid JSON, whereas {"key": "Some 'value'"} is.

They are equivalent for all intents and purposes. If you want to use either one inside a string, it is a good idea to use the other one to create the string, as you noted. Other than that, it's all the same.

There's no difference. The reason for its existence is exactly what you mentioned

Good practice, according to Mozilla, is to use " " in HTML (where ' ' cannot be used) while reserving ' ' in Javascript (where both " " and ' ' can be use indifferently)...