Searching with indexOf

Learn to find an element's position with indexOf.

Finding an element position with indexOf

The indexOf method returns the first position (index) of an element in the array. If it cannot find it, it returns -1.

Example: ["apple", "pear", "banana"].indexOf("pear") returns 1. The search goes front to back; the first match is returned.

A start index can be passed as a second argument: arr.indexOf(x, 2) starts searching from index 2.

When not found, -1

When indexOf cannot find the element it returns -1. So to check whether an element exists, you write if (arr.indexOf(x) !== -1).

Example: ["a", "b"].indexOf("z") returns -1 because "z" is not in the array.

This pattern is common, but newer code has a more readable alternative: the includes method returns true/false directly.

Exercise Find a city position in the cities array

Create an array named cities with at least four city strings. Use cities.indexOf to find a city's position and print the result with console.log.

Show example solution

Write the array, then search for a city with cities.indexOf and print the result with console.log.
Example solution:
const cities = ["Istanbul", "Ankara", "Izmir", "Bursa"];
const pos = cities.indexOf("Izmir");
console.log(pos);

Test yourself

What does this code print?
```
const fruits = ["apple", "pear", "banana"];
console.log(fruits.indexOf("banana"));
```
- 1
- 3
- 2
- -1
Show answer

2 — Correct. "banana" is at index 2; indexOf returns 2.

Find the position of blue

const colors = ["red", "blue", "green"];
const pos = colors.indexOf(____);
console.log(pos);

Show answer

const colors = ["red", "blue", "green"];
const pos = colors.indexOf("blue");
console.log(pos);

Match indexOf behaviors

indexOf when not found

returns -1
indexOf first match

returns its index
indexOf second argument

start index

Next lesson: Checking with includes

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