JavaScript Array Methods, Under the Hood, Part

This article continues our exploration of JavaScript array methods. If you're new to this series, you can find Part 1 here. We'll examine several common array methods—Math.max(), Math.min(), .reverse(), .filter(), and .reduce()—and show how to achieve the same results using for loops. This comparison will hopefully clarify how these methods function.

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Math.min() and Math.max()

These methods find the smallest and largest numbers in a set, respectively. You can provide numbers directly as arguments or use an array with the spread syntax.

// FIND THE MINIMUM VALUE IN AN ARRAY:
// Using a 'for' loop:
function findMinValueInArray(array) {
  let min = array[0];
  for (let i = 1; i < array.length; i++) {
    if (array[i] < min) {
      min = array[i];
    }
  }
  return min;
}
findMinValueInArray([33, -5544, 66, 55]); // returns: -5544

// Using Math.min():
function findMinValueInArray(array) {
  return Math.min(...array);
}
findMinValueInArray([33, -5544, 66, 55]); // returns: -5544

Math.min() is clearly more concise and less error-prone. Math.max() works similarly, but its for loop implementation differs slightly:

// FIND THE MAXIMUM VALUE IN AN ARRAY:
// Using a 'for' loop:
function findMaxValueInArray(array) {
  let max = array[0];
  for (let i = 1; i < array.length; i++) {
    let currentNum = array[i];
    if (currentNum > max) {
      max = currentNum;
    }
  }
  return max;
}
findMaxValueInArray([33, -5544, 66, 55]); // returns: 66

// Using Math.max():
function findMaxValueInArray(array) {
  return Math.max(...array);
}
findMaxValueInArray([33, -5544, 66, 55]); // returns: 66

`.reverse()`

This method reverses the order of elements in an array, modifying the original array.

// REVERSE ARRAY ELEMENTS
// Using a 'for' loop:
function reverseArray(array) {
  let reversedArray = [];
  for (let i = array.length - 1; i >= 0; i--) {
    reversedArray.push(array[i]);
  }
  return reversedArray;
}
reverseArray([1, 2, 3, 4, 5]); // returns: [5, 4, 3, 2, 1]

// Using .reverse():
function reverseArray(array) {
  return array.reverse();
}
reverseArray([1, 2, 3, 4, 5]); // returns: [5, 4, 3, 2, 1]

The built-in method is significantly more efficient and readable. The for loop approach requires creating a new array and manually pushing elements, increasing the risk of errors.

`.filter()`

This method creates a new array containing only elements that pass a provided test function.

Let's consider an array of bank accounts, each with a balance property. We'll filter for accounts with a zero balance.

// Using a 'for' loop:
function getClientWithNoMoney(array) {
  let acctsWithZeroBalance = [];
  for (let account of array) {
    if (account.balance === 0) {
      acctsWithZeroBalance.push(account);
    }
  }
  return acctsWithZeroBalance;
}

// Using .filter():
function getClientWithNoMoney(array) {
  return array.filter((account) => account.balance === 0);
}

.filter() provides a much cleaner and more concise solution. The for loop requires manual array creation and element pushing, making it more prone to mistakes.

`.reduce()`

.reduce() applies a function cumulatively to the array elements, resulting in a single value.

A simple example is summing array elements, but let's calculate the average:

// With 'for' loop:
function getAverage(array) {
  let sum = 0;
  for (let i = 0; i < array.length; i++) {
    sum += array[i];
  }
  return sum / array.length;
}

// Using .reduce():
function getAverage(array) {
  return array.reduce((cv, acc) => {
      return cv + acc;
    }) / array.length;
  );
}
getAverage([10, 20, 30, 40]); // returns: 25

While the code savings aren't dramatic here, .reduce() still improves readability and reduces the potential for errors compared to the for loop approach.

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These examples illustrate the advantages of using built-in JavaScript array methods. While for loops provide fundamental control, the methods offer conciseness, readability, and reduced error potential. If you have questions about other array methods, please leave a comment. Share this article if you found it helpful! Part 1 of this series is available here.

Thanks for reading!

Originally published to Medium for JavaScript in Plain English on February 13, 2023

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