String .indexOf() / lastIndexOf() / includes()

string.indexOf("searchString")
string.indexOf("searchString", startPosition)

string.lastIndexOf("searchString")
string.lastIndexOf("searchString", startPosition)

// Returns true or false depending on whether the string is found.
string.includes("searchString")
string.includes("searchString", startPosition)

var str = "JavaScript is great. JavaScript is fun.";

console.log(str.indexOf("JavaScript")); // Outputs 0.
console.log(str.indexOf("is")); // Outputs 11.
console.log(str.indexOf("Python")); // Not found, so outputs -1.

console.log(str.indexOf("JavaScript", 1)); // Outputs 21.

console.log(str.lastIndexOf("JavaScript")); // Outputs 21.

console.log(str.includes("great")); // Outputs true.
console.log(str.includes("Python")); // Outputs false.

// Pattern 2: Array indexOf / includes
var members = ["item_a", "item_b", "item_c", "item_d", "item_e"];

console.log(members.indexOf("item_b")); // Outputs 1.
console.log(members.indexOf("item_z")); // Not found, so outputs -1.

// To check existence, compare the result with -1.
var exists = members.indexOf("item_c") !== -1;
console.log(exists); // Outputs true.

// Pattern 3: Using the indexOf result to operate on an element (combined with splice)
var list = ["item_a", "item_b", "item_c"];
var target = "item_b";
var pos = list.indexOf(target);
if (pos !== -1) {
	list.splice(pos, 1); // Remove the found element.
}
console.log(list); // Outputs "item_a,item_c".

// Pattern 4: Use lastIndexOf to find the last occurrence of a duplicate value
var scores = [80, 90, 75, 90, 60];
console.log(scores.indexOf(90)); // Outputs 1 — the first occurrence.
console.log(scores.lastIndexOf(90)); // Outputs 3 — the last occurrence.

0
11
-1
21
21
true
false
1
-1
true
[ 'item_a', 'item_c' ]
1
3

string.indexOf() and string.lastIndexOf() are methods that search for a specified string within a string and return its position (index) as a number. string.indexOf() searches from the beginning, while string.lastIndexOf() searches from the end. Both return -1 if the string is not found.

When you only need to check whether a string is present, string.includes() returns a simple true or false, which makes your code easier to read. With string.indexOf(), you need to compare against -1, as in str.indexOf("abc") !== -1, whereas string.includes() lets you write the same check as simply str.includes("abc").

All of these methods are case-sensitive. If you need a case-insensitive search, convert the string to lowercase first using string.toLowerCase() before searching.