JavaScript Loop [forEach, map, reduce, each,filter]

ECMAScript5 (the version on Javascript) to work with Arrays.

forEach - Iterates through every item in the array and do whatever you need with each item.

['C', 'D', 'E'].forEach(function(element, index) {
  console.log(element + " is the #" + (index+1) + " in musical scale");
});

// Output
// C is the #1 in musical scale
// D is the #2 in musical scale
// E is the #3 in musical scale

In case , more interested on operation on array using some inbuilt feature.

map - It creates a new array with the result of the callback function. This method is good to be used when you need to format the elements of your array.

// Let's upper case the items in the array
['bob', 'joe', 'jen'].map(function(elem) {
  return elem.toUpperCase();
});

// Output: ['BOB', 'JOE', 'JEN']

reduce - As the name says it reduces the array to a single value by calling the given function passing in the currenct element and the result of the previous execution.

[1,2,3,4].reduce(function(previous, current) {
  return previous + current;
});
// Output: 10
// 1st iteration: previous=1, current=2 => result=3
// 2nd iteration: previous=3, current=3 => result=6
// 3rd iteration: previous=6, current=4 => result=10

every - Returns true or false if all the elements in the array pass the test in the callback function.

// Check if everybody has 18 years old of more.
var ages = [30, 43, 18, 5];  
ages.every(function(elem) {  
  return elem >= 18;
});

// Output: false

filter - Very similar to every except that filter return an array with the elements that return true to the given function.

// Finding the even numbers
[1,2,3,4,5,6].filter(function(elem){
  return (elem % 2 == 0)
});

// Output: [2,4,6]