试试看：

function countString(str, search){
    var count=0;
    var index=str.indexOf(search);
    while(index!=-1){
        count++;
        index=str.indexOf(search,index+1);
    }
    return count;
}

正则表达式（global的缩写）中的g表示搜索整个字符串，而不仅仅是查找第一个出现的字符串。此匹配是两次：

var temp=“这是一个字符串。”；var count=（temp.match（/is/g）| |[]）.length；console.log（计数）；

如果没有匹配项，则返回0：

var temp=“Hello World！”；var count=（temp.match（/is/g）| |[]）.length；console.log（计数）；

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)

       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。

var mystring = 'This is the lorel ipsum text';
var mycharArray = mystring.split('');
var opArr = [];
for(let i=0;i<mycharArray.length;i++){
if(mycharArray[i]=='i'){//match the character you want to match
    opArr.push(i);
  }}
console.log(opArr); // it will return matching index position
console.log(opArr.length); // it will return length

只需编码打高尔夫球丽贝卡·切尔诺夫的解决方案：-）

alert(("This is a string.".match(/is/g) || []).length);