我们可以使用jssplit函数，它的长度减1就是出现的次数。

var temp = "This is a string.";
alert(temp.split('is').length-1);

基于@Vittim.us的上述回答。我喜欢他的方法给我的控制，使其易于扩展，但我需要添加不区分大小写的功能，并将匹配限制在支持标点符号的整个单词中。（例如，“洗澡”是指“洗澡”，而不是“洗澡”）

标点正则表达式来自：https://stackoverflow.com/a/25575009/497745（如何使用正则表达式从JavaScript字符串中删除所有标点符号？）

function keywordOccurrences(string, subString, allowOverlapping, caseInsensitive, wholeWord)
{

    string += "";
    subString += "";
    if (subString.length <= 0) return (string.length + 1); //deal with empty strings

    if(caseInsensitive)
    {            
        string = string.toLowerCase();
        subString = subString.toLowerCase();
    }

    var n = 0,
        pos = 0,
        step = allowOverlapping ? 1 : subString.length,
        stringLength = string.length,
        subStringLength = subString.length;

    while (true)
    {
        pos = string.indexOf(subString, pos);
        if (pos >= 0)
        {
            var matchPos = pos;
            pos += step; //slide forward the position pointer no matter what

            if(wholeWord) //only whole word matches are desired
            {
                if(matchPos > 0) //if the string is not at the very beginning we need to check if the previous character is whitespace
                {                        
                    if(!/[\s\u2000-\u206F\u2E00-\u2E7F\\'!"#$%&\(\)*+,\-.\/:;<=>?@\[\]^_`{|}~]/.test(string[matchPos - 1])) //ignore punctuation
                    {
                        continue; //then this is not a match
                    }
                }

                var matchEnd = matchPos + subStringLength;
                if(matchEnd < stringLength - 1)
                {                        
                    if (!/[\s\u2000-\u206F\u2E00-\u2E7F\\'!"#$%&\(\)*+,\-.\/:;<=>?@\[\]^_`{|}~]/.test(string[matchEnd])) //ignore punctuation
                    {
                        continue; //then this is not a match
                    }
                }
            }

            ++n;                
        } else break;
    }
    return n;
}

如果发现错误或改进，请随时修改和重构此答案。

       var myString = "This is a string.";
        var foundAtPosition = 0;
        var Count = 0;
        while (foundAtPosition != -1)
        {
            foundAtPosition = myString.indexOf("is",foundAtPosition);
            if (foundAtPosition != -1)
            {
                Count++;
                foundAtPosition++;
            }
        }
        document.write("There are " + Count + " occurrences of the word IS");

请参阅：-count字符串中出现的子字符串，以了解分步说明。

const getLetterMatchCount = (guessedWord, secretWord) => {
  const secretLetters = secretWord.split('');
  const guessedLetterSet = new Set(guessedWord);
  return secretLetters.filter(letter => guessedLetterSet.has(letter)).length;
};
const str = "rahul";
const str1 = "rajendra";

getLetterMatchCount(str, str1)

String.prototype.Count=函数（查找）{返回this.split（find）.length-1；}console.log（“这是一个字符串。”.Count（“是”））；

这将返回2。

我们可以使用jssplit函数，它的长度减1就是出现的次数。

var temp = "This is a string.";
alert(temp.split('is').length-1);