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Finding Array Elements

Learning Goals

  • Find elements using Array.prototype.find()
  • Find elements using Array.prototype.indexOf()

Introduction

Use Case Method
Executing a provided callback on each element forEach()
** Finding a single element that meets a condition find(), indexOf()
Finding and returning a list of elements that meet a condition filter()
Modifying each element and returning the modified array map()
Creating a summary or aggregation of values in an array reduce()

In the last lesson, we learned about the forEach() iterator method. In this lesson we'll look at two iterator methods that help us locate data in arrays: Array.prototype.find(), which can be used for a wide array of search conditions, and Array.prototype.indexOf(), which can be used to search for a specific value.

As you work through this lesson, you will notice that there is a lot of overlap in how forEach() and find() work (indexOf() is a bit different). This will be the case for all of the iterator methods you will learn about in this module other than indexOf(). Specifically, they all:

  1. iterate through the elements in the array they're called on,
  2. take a callback as an argument,
  3. automatically pass values to the callback, and
  4. (with the exception of forEach()) return a value at the end.

All of them except reduce() pass the same three values to the callback in step 3 above: the current element, the index of the current element, and the array itself. reduce() is a bit more complicated, but we'll get to that later.

Find Elements Using Array.prototype.find()

Array.prototype.find() is called on an array and takes a callback function as an argument. The method will automatically iterate through the array, call the callback on each value, and return the first element in the array that satisfies the condition defined by the callback. If no matching element is found, undefined is returned.

In each iteration through the array, Array.prototype.find() passes three arguments to the callback: the current element of the array, the index of the current element, and the array itself. These arguments can then be captured as parameters in the callback and used inside the function.

Say we want to determine whether an array of numbers contains any odd values. We can write the following callback function to do this:

function isOdd(element, index, array) {
  return element % 2 === 1;
}

Array.prototype.find() will iterate through the array it's called on, passing the element (and the other two values) in turn to isOdd(). If the element is not odd, the callback returns false and the iteration continues. If an odd element is encountered, the callback will return true, and Array.prototype.find() will return that element.

Remember that Array.prototype.find() automatically passes the three arguments to our function. By defining isOdd() with three parameters, we make those values available inside our function. In this example, we're only using the first one, the current element of the array, but all three are being passed in and are available inside our function if we want to use them.

Let's call .find() on the array we want to search, and pass our function as an argument:

function isOdd(element, index, array) {
  return element % 2 === 1;
}

[4, 6, 8, 10].find(isOdd); //=> undefined, not found
[4, 5, 8, 10].find(isOdd); //=> 5
[4, 5, 7, 8, 10].find(isOdd); //=> 5
[4, 7, 5, 8, 10].find(isOdd); //=> 7

Let's walk through what's happening here, using the last example:

  1. We call .find() on the array [4, 7, 5, 8, 10] and pass the isOdd() function as an argument.
  2. The .find() method calls the callback and passes it the three arguments, in this case 4, 0, and the array itself.
  3. isOdd() checks whether the current element is odd. It isn't, so it returns false.
  4. .find() continues to the next element in the array and calls the callback again, passing it the new set of arguments.
  5. isOdd() checks whether the current element (7) is odd and returns true.
  6. .find() returns the current element and ends execution.

Note that in this case only the first argument — the current element in the array — is required for the callback function. Recall from the last lesson that, if (as in our example above) your callback doesn't use the other two arguments, you can define your function with only the first parameter. This will work as well:

function isOdd(element) {
  return element % 2 === 1;
}

The ability to pass a callback function to .find() makes the method very versatile. Anything you can program that returns a Boolean value can be used as the search condition: a number greater than or less than a certain value, a value of a certain data type, a word that begins with a vowel — the sky's the limit! But sometimes you just need to determine whether a particular value is present in the array. In this case, Array.prototype.indexOf() will save you some work.

Find Elements Using a Simple Condition with Array.prototype.indexOf()

If you need to search an array for a value, Array.prototype.indexOf() is a simpler option. It is called on an array and takes two arguments: the value you are looking for and an optional start position — no need to write a callback function! It compares each element in turn to the value you're looking for using the strict equality operator (===) and returns the index of the first matching element. If the element isn't contained in the array, it returns -1.

const cards = [
  "queen of hearts",
  "jack of clubs",
  "ten of diamonds",
  "ace of spades",
];

cards.indexOf("jack of clubs"); //=> 1
cards.indexOf("jack of hearts"); //=> -1

If you pass in the optional second argument, indexOf() will begin the search at the specified position:

cards.indexOf("ace of spades", 2); //=> 3
cards.indexOf("jack of clubs", 2); //=> -1

In this case, Array.prototype.indexOf() returns -1 if either the value isn't found or if the start position you pass in is after the element you're looking for.

Challenge: For practice, let's consider how we could use the find() method rather than indexOf() to find a specific value, say, "king of diamonds".

What would the callback function be? function isKingOfDiamonds(element) {
return element === "king of diamonds"
}
What would `find()` return if the value is in the array?
  • 'king of diamonds'
What would `find()` return if the value isn't in the array?
  • undefined

Conclusion

Both Array.prototype.indexOf()and Array.prototype.find() can be very useful in different situations. Array.prototype.indexOf() is used when you want to check an array for a simple value; you call indexOf() on an array, passing the value you're looking for as the argument. Array.prototype.find() is also called on an array, but it takes a function as an argument. This enables you to define the condition the element should meet, allowing for more complex searches.

Resources