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forEach 활용

const char = 'a';
const str = 'aaabbbccc';

let count = 0;
[...str].forEach((ele) => {
  if (ele === char) {
    count++;
  }
});
console.log(count); // 3

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reduce 함수 활용

const char = 'a';
const str = 'aaabbbccc';

const answer = [...str].reduce((acc, cur) => (cur === char ? acc + 1 : acc), 0);
console.log(answer); // 3

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match 함수와 정규식 활용

const char = 'a';
const str = 'aaabbbccc';
const regex = new RegExp(char, 'g');
console.log((str.match(regex) || []).length); // 3