Clarify the prototype chain of foo and F, find A and B attributes along the prototype chain;
console.log(foo.a)
foo is an object The prototype chain of the object is Object
foo.a has a on the prototype chain, so the output value a
console.log(foo.b) undefined
There is no F on the prototype chain of foo
console.log(F.a) value a
The prototype chain of F is the function prototype. The function prototype points to the Object prototype.
console.log(F.b) value b
The prototype chain of F is the function prototype function prototype