Classic problem of hoisting.

hoisting.js

var foo = 1; 

function bar(){ 
  if(!foo){ 
    var foo = 10; 
  } 
  alert(foo); 
} 

bar();


// HOISTING
// Converts to

function bar(){ 
  var foo;   // local scope foo
  if(!foo){  // considers local scope foo
    foo = 10; 
  } 
  alert(foo);  // gives value of local scope foo i.e 10
} 
var foo;  // defines global scope foo
foo = 1;  // assign value to global scope foo
bar();    // call the function, gives value of local scope foo i.e 10
// alert(foo); => gives value of global scope foo i.e 1