javascript - Why does my child function require a 'return' statement to avoid erroring out? The parent function has no 'return' and no error -

var = function(param1) {     var b = function(param2) {         console.log(param1 + param2);     };      return b; };  a(4)(5); // logs "9" 

is return statement (in case "return b") necessary? why need 1 function here, not other? when necessary, , why?

the expression

a(4)(5) 

first calls function a parameter 4 (a(4)), returns b (which function). returned function called parameter 5. same if did

c = a(4);  c(5); 

so, need outer function a return function - because treat it's result function. , don't care if inner function b returns don't use result, doesn't matter if returns anything.