JavaScript Variable Undefined Scope

Question

var a = 1;
var b = 2;
var c = 3;
(function firstFunction() {
   console.log("a: " + a + ", b: " + b + ", c: " + c);
   var b = 5, c = 6;
   console.log("a: " + a + ", b: " + b + ", c: " + c);
   (function secondFunction() {
      var b = 8;
      console.log("a: " + a + ", b: " + b + ", c: " + c);
      (function thirdFunction() {
         var a = 7; c = 9;
         (function fourthFunction() {
            var a = 1, c = 8;
         })();
      })();
   })();
})();

a: 1, b: undefined, c: undefined a: 1, b: 5, c: 6 a: 1, b: 8, c: 6

Why is a = 1 on the first console.log but b and c are undefined?

Thank you for any help and explanation.