Count how many times a character appears in a string.

    var greeting = "Hello world!";

    var hash = {};
    for(var i = 0; i < greeting.length; i++)
    {
      if(hash[greeting[i]] === undefined)
      {
        hash[greeting[i]] = 1;
      } else {
        hash[greeting[i]] += 1;
      }
    }

    // printing the stuff in hash.
    for(var x in hash)
    {
      if(hash.hasOwnProperty(x))
      {
        console.log(x, hash[x]);
      }
    }

    Output:
    H 1
    e 1
    l 3
    o 2
      1
    w 1
    r 1
    d 1
    ! 1

We can possibly convert the code into functions.

    var generateCharacterHashOfString = function(str)
    {
        var hash = {};
        for(var i = 0; i < greeting.length; i++)
        {
          if(hash[greeting[i]] === undefined)
          {
            hash[greeting[i]] = 1;
          } else {
            hash[greeting[i]] += 1;
          }
        }
        return hash;
    };

And the print function:

    var printCharacterHash = function(hash)
    {
        for(var x in hash)
        {
          if(hash.hasOwnProperty(x))
          {
            console.log(x, hash[x]);
          }
        }
    };

Then it's just a matter of calling these functions:

    var hash = generateCharacterHashOfString("Hello world!");
    printCharacterHash(hash);