Using Array.indexOf() to Compare Multiple Items in JavaScript

We are all familiar with the Logical OR Operator (||). One of the most common uses of the OR operator is to compare one value to several other values in a condition.

Let’s look at an example: Write a function that takes the name of a month as an argument and returns whether that month has 31 days or not.

function checkMonth(month)
if (month === 'January' || month === 'March' ||
   month === 'May' || month === 'July' ||
   month === 'August' || month === 'October' || month = 'December') {
   return console.log(`${month} has 31 days.`)
} else {
   return console.log(`${month} doesn't have 31 days.`)
}
checkMonth('July') // 'July has 31 days'
checkMonth('February') // 'February doesn't have 31 days'

Here, we are using the or operator five times to get the result, and there is repetitive code, which doesn’t look clean. When we compare 2 or 3 values using the or operator, the code doesn’t look too bad. However, as the number of values increases, the code looks messy.

As developers, we should all follow the most important rule for writing code: DRY (Don’t Repeat Yourself). Never repeat your code. So, we need to eliminate these repetitive statements, and this is where the Array.indexOf() method comes in. Before solving the problem using the Array.indexOf() method, let’s understand what this method does.

The Array.indexOf() Method

The .indexOf() method takes two arguments: the element we are searching for and the index from which we start searching. You can pass either just the first argument or both arguments.

• If we pass the element we are searching for as the only argument, the method will return the first index where the element is found. If the element is not in the array, it will return -1.

• If we pass both arguments, this method will search for the given element starting from the specified index in the array.

Let’s go through some examples for a better understanding:

const animals = ['tiger', 'Lion', 'monkey', 'dog', 'cat', 'Lion', 'tiger'];

// Passing only one argument
console.log(animals.indexOf('lion')); // output = 1
console.log(animals.indexOf('cow')) // output = -1

// Passing both arguments
console.log(animals.indexOf('lion', 2)); // output = 5
console.log(animals.indexOf('monkey', 3)); // output -1, since element is not present after index 3

Implementation

Now that we understand the .indexOf() method, let’s rewrite the previous function.

function checkMonth(month) {
   const compareMonths = ['January', 'March', 'May', 'July', 'August', 'October', 'December'];

   // We can use ternary instead of if...else
   return compareMonths.indexOf(month) ? `${month} has 31 days` : `${month} doesn't have 31 days`;
}

console.log(checkMonth('July')); // "July has 31 days"
console.log(checkMonth('February')) // "February doesn't have 31 days"

I hope you found this article helpful. For more details about the indexOf() method, visit the MDN docs.