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

1/** Function that count occurrences of a substring in a string;
2 * @param {String} string               The string
3 * @param {String} subString            The sub string to search for
4 * @param {Boolean} [allowOverlapping]  Optional. (Default:false)
5 *
6 * @author Vitim.us https://gist.github.com/victornpb/7736865
7 * @see Unit Test https://jsfiddle.net/Victornpb/5axuh96u/
8 * @see http://stackoverflow.com/questions/4009756/how-to-count-string-occurrence-in-string/7924240#7924240
9 */
10function occurrences(string, subString, allowOverlapping) {
11
12    string += "";
13    subString += "";
14    if (subString.length <= 0) return (string.length + 1);
15
16    var n = 0,
17        pos = 0,
18        step = allowOverlapping ? 1 : subString.length;
19
20    while (true) {
21        pos = string.indexOf(subString, pos);
22        if (pos >= 0) {
23            ++n;
24            pos += step;
25        } else break;
26    }
27    return n;
28}