Check if a String contains a Substring in JavaScript

Title : Check if a String contains a Substring in JavaScript

1. Check if a String contains a Substring with String.includes():

const str1 = "Hello, world!";
const substring1 = "world";

const containsSubstring1 = str1.includes(substring1);
console.log(containsSubstring1); // Output: true

Explanation:

We use the includes() method on the string str1 to check if it contains the substring "world".
Since "world" is a part of "Hello, world!", includes() returns true.

2. Check if String Doesn't contain a Substring in JavaScript:

const str2 = "Hello, world!";
const substring2 = "foo";

const doesNotContainSubstring2 = !str2.includes(substring2);
console.log(doesNotContainSubstring2); // Output: true

Explanation:

We use the includes() method to check if the string str2 contains the substring "foo".
Since "foo" is not a part of "Hello, world!", includes() returns false.
Using the logical NOT (!) operator, we negate the result, which gives us true.

3. Check if a String contains a Substring with String.indexOf():

const str3 = "Hello, world!";
const substring3 = "world";

const containsSubstring3 = str3.indexOf(substring3) !== -1;
console.log(containsSubstring3); // Output: true

Explanation:

We use the indexOf() method on the string str3 to check if it contains the substring "world".
indexOf() returns the index of the substring in the string if found, otherwise -1.
Since the substring "world" is found in "Hello, world!" at index position 7, indexOf() doesn't return -1, so the condition evaluates to true.

4. Check if String Doesn't contain a Substring using indexOf():

const str4 = "Hello, world!";
const substring4 = "foo";

const doesNotContainSubstring4 = str4.indexOf(substring4) === -1;
console.log(doesNotContainSubstring4); // Output: true

Explanation:

We use the indexOf() method to get the index of the substring "foo" in the string str4.
Since "foo" is not a part of "Hello, world!", indexOf() returns -1.
We check if the returned value is equal to -1 to determine if the string doesn't contain the substring, which evaluates to true.

5. Using includes() and indexOf() with case sensitivity:

const str5 = "Hello, world!";
const substring5 = "WORLD";

const containsSubstringCaseSensitive = str5.includes(substring5);
console.log(containsSubstringCaseSensitive); // Output: false

const containsSubstringIgnoreCase = str5.toLowerCase().includes(substring5.toLowerCase());
console.log(containsSubstringIgnoreCase); // Output: true

Explanation:

By default, both includes() and indexOf() are case-sensitive.
In the first example, "WORLD" is not found in "Hello, world!" because of the different case, so includes() returns false.
In the second example, we convert both the string and the substring to lowercase before checking, so "world" is found in "hello, world!", and includes() returns true.