Searching an Array

Is a name on the guest list? Where in the queue is someone standing? Two methods answer questions like these.

The method includes() finds out whether an array contains a value. It returns true if it does and false if it doesn't:

const guests = ["Anna", "Tom", "Lucy"];
console.log(guests.includes("Tom"));
console.log(guests.includes("Paul"));

This prints:

true
false

A boolean is exactly what a condition needs, so includes() is most often used with if:

const guests = ["Anna", "Tom", "Lucy"];
const visitor = "Tom";
if (guests.includes(visitor)) {
    console.log("Come in!");
} else {
    console.log("Sorry, you are not on the list.");
}

This prints:

Come in!

The method indexOf() tells you where a value is: it returns its index. If the value isn't in the array, it returns -1. Why -1? Because indexes start at 0, so -1 can never be a real index:

const queue = ["Anna", "Tom", "Lucy"];
console.log(queue.indexOf("Lucy"));
console.log(queue.indexOf("Paul"));

This prints:

2
-1

So the condition queue.indexOf("Paul") === -1 means “Paul isn't in the queue”. For a simple yes-or-no question, though, includes() reads better.

Both methods compare exactly, like ===. guests.includes("tom") returns false, because "tom" with a lowercase t isn't the same text as "Tom".