arr.forEach(callback(currentvalue[, index[, array]])[, thisArg])
์ฐธ๊ณ ) [] ๋ optional
const nations = ["korea", "usa", "china", "japan"];
nations.forEach((nation) => {
console.log(nation);
});
// korea
// usa
// china
// japan
let idxOfC = -1;
let arr = ['a', 'b', 'c', 'd'];
arr.forEach((el, idx) => {
if (el === 'c') {
idxOfC = idx;
return;
}
});