4 ms·

Do you have a concrete example of such a time?

• schwartzworld 6y ago
  Undefined represents the absence of information altogether. In JavaScript any key can be called on any object and if it doesn't exist it will return undefined. If you put a typo in the key name, you get undefined. If you've never set it, you get undefined. Null, on the other hand, is not a lack of information. It's an absolute value just as much so as any number or string or array. They're both falsey values, but so is the number zero. An item with a price of zero dollars is not the same as an item with an undefined price, nor is it the same as an item that explicitly has no price, i.e. null. You may not choose to model your data that way, but the terms are still meaningful, and your UI might have to change to reflect the different falsey values. Most common case I could think of, would be in making a request from a server, using something like Apollo. `const [ data ] = useQuery(SomeQuery)` The data that comes from the server is undefined until you make the request, but null could be a valid response.