1function count(str, find) {
2    return (str.split(find)).length - 1;
3}
4
5count("Good", "o"); // 2

1var temp = "This is a string.";
2var count = (temp.match(/is/g) || []).length;
3console.log(count);
4
5Output: 2
6
7Explaination : The g in the regular expression (short for global) says to search the whole string rather than just find the first occurrence. This matches 'is' twice.

1function countOccurences(string, word) {
2   return string.split(word).length - 1;
3}
4var text="We went down to the stall, then down to the river."; 
5var count=countOccurences(text,"down"); // 2

1function count (string) {  
2  var count = {};
3  string.split('').forEach(function(s) {
4     count[s] ? count[s]++ : count[s] = 1;
5  });
6  return count;
7}

1 const recorrences = ['a', 'b', 'c', 'a', 'b','a']
2                .map(i => !!~i.indexOf('a'))
3                .filter(i => i)
4                .length;
5console.log(`recorrences ${recorrences}`) 
6//recorrences 3

1 function countInstancesOf(letter, sentence) {
2  var count = 0;
3
4  for (var i = 0; i < sentence.length; i++) {
5    if (sentence.charAt(i) === letter) {
6      count += 1;
7    }
8  }
9  return count;
10}