Open the console (Cmd+option+J on Chrome) and Refresh
You'll what's going on step by stepvar numbers = new Array(3); // Create array with 3 undefined elements
console.log(numbers);    
numbers[0] = 90; 
numbers[1] = 150; 
numbers[2] = 30; 
var a = numbers[0] + numbers[1]; // Sets variable 'a' to 240 
var b = numbers[1] + numbers[2]; // Sets variable 'b' to 180
console.log(numbers);
console.log("a=%d and b=%d", a,b);