Set JavaScript variable = null, or leave undefined?

It depends on the context.

• "undefined" means this value does not exist. typeof returns "undefined"

• "null" means this value exists with an empty value. When you use typeof to test for "null", you will see that it's an object. Other case when you serialize "null" value to backend server like asp.net mvc, the server will receive "null", but when you serialize "undefined", the server is unlikely to receive a value.

I declare them as undefined when I don't assign a value because they are undefined after all.

Generally, I use null for values that I know can have a "null" state; for example

if(jane.isManager == false){
  jane.employees = null
}

Otherwise, if its a variable or function that's not defined yet (and thus, is not "usable" at the moment) but is supposed to be setup later, I usually leave it undefined.

Generally speak I defined null as it indicates a human set the value and undefined to indicate no setting has taken place.