<<javascript the good parts>> variable scope

Javascript does not have block scope, but it does have function scope. A variable defined anywhere within a function is visible everywhere within the function.

 

function foo(){
  console.log("a:",a);//variable a is visible here but it is still undefined
  console.log("f:",f);//variable f is visible here but it is still undefined
  var a=1;
  var f=function(){
    console.log("f");
  }
}

foo();

 

So, it is best to declare all of the variables used in a function at the top of the function body. If we check the angularjs source code, we find it does exactly the same way.

// at the top, it declares all the global variables
var $$scope           = '$scope',
    $boolean          = 'boolean',
    $console          = 'console',
    $length           = 'length',
    $name             = 'name',
    $object           = 'object',
...

// in each function, it also declare the variables at the top 
function forEach(obj, iterator, context) {
  var key;
  if (obj) {...
...