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To check whether a string contains a substring in JavaScript, you can use the includes() method of the String object. The includes() method returns a boolean value indicating whether the string contains the specified substring.

Here’s an example of how to use the includes() method to check for a substring:

const str = 'The quick brown fox jumps over the lazy dog.';
console.log(str.includes('fox')); // true
console.log(str.includes('cat')); // false

You can also use the indexOf() method to check for a substring. The indexOf() method returns the index of the substring within the string, or -1 if the substring is not found.

Here’s an example of how to use the indexOf() method to check for a substring:

const str = 'The quick brown fox jumps over the lazy dog.';
if (str.indexOf('fox') !== -1) {
  console.log('The string contains the substring');
} else {
  console.log('The string does not contain the substring');
}

Keep in mind that both the includes() and indexOf() methods are case-sensitive, so they will not match substrings that have a different case.

const str = 'The quick brown fox jumps over the lazy dog.';
console.log(str.includes('Fox')); // false
console.log(str.indexOf('Fox')); // -1