String.prototype.includes()

const sentence = "The quick brown fox jumps over the lazy dog.";
const word = "fox";
console.log(
  `The word "${word}" ${
    sentence.includes(word) ? "is" : "is not"
  } in the sentence`,
);
// 予想される結果: "The word "fox" is in the sentence"

includes(searchString)
includes(searchString, position)

"Blue Whale".includes("blue"); // false を返す

"Blue Whale".toLowerCase().includes("blue"); // true を返す

const str = "To be, or not to be, that is the question.";
console.log(str.includes("To be")); // true
console.log(str.includes("question")); // true
console.log(str.includes("nonexistent")); // false
console.log(str.includes("To be", 1)); // false
console.log(str.includes("TO BE")); // false
console.log(str.includes("")); // true