Yogesh Chavan

Posted on Feb 19, 2021 • Updated on Apr 26, 2021 • Originally published at freecodecamp.org

The Most Useful JavaScript Array Methods Explained with Examples

#javascript #react #code

If you're a JavaScript developer and want to improve your coding, then you should be familiar with the most commonly used ES5 and ES6+ array methods.

These methods make coding a lot easier and also make your code look clean and easy to understand.

So in this article, we will explore some of the most popular and widely used array methods. So let's get started.

The Array.forEach Method

The Array.forEach method has the following syntax:

Array.forEach(callback(currentValue [, index [, array]])[, thisArg]);

The forEach method executes a provided function once for every element in the array.

Take a look at the below code:

const months = ['January', 'February', 'March', 'April'];

months.forEach(function(month) {
  console.log(month);
});

/* output

January
February
March
April

*/

Here's a Code Pen Demo.

Here, inside the forEach loop callback function, each element of the array is automatically passed as the first parameter of the function.

The equivalent for loop code for the above example looks like this:

const months = ['January', 'February', 'March', 'April'];

for(let i = 0; i < months.length; i++) {
  console.log(months[i]);
}

/* output

January
February
March
April

*/

Here's a Code Pen Demo.

The thing you need to keep in mind is that the forEach method does not return any value.

Take a look at the below code:

const months = ['January', 'February', 'March', 'April'];
const returnedValue = months.forEach(function (month) {
  return month;
});

console.log('returnedValue: ', returnedValue); // undefined

Here's a Code Pen Demo.

Note that forEach is only used to loop through the array and perform some processing or logging. It does not return any value, even if you explicitly return a value from the callback function (this means that the returned value comes as undefined in the above example).

In all the above examples, we have used only the first parameter of the callback function. But the callback function also receives two additional parameters, which are:

index - the index of the element which is currently being iterated
array - original array which we're looping over

const months = ['January', 'February', 'March', 'April'];

months.forEach(function(month, index, array) {
  console.log(month, index, array);
});

/* output

January 0 ["January", "February", "March", "April"]
February 1 ["January", "February", "March", "April"]
March 2 ["January", "February", "March", "April"]
April 3 ["January", "February", "March", "April"]

*/

Here's a Code Pen Demo.

Depending on the requirement, you may find it useful to use the index and array parameters.

Advantages of using forEach instead of a for loop

Using a forEach loop makes your code shorter and easier to understand
When using a forEach loop, we don't need to keep track of how many elements are available in the array. So it avoids the creation of an extra counter variable.
Using a forEach loop makes code easy to debug because there are no extra variables for looping through the array
The forEach loop automatically stops when all the elements of the array are finished iterating.

Browser Support

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

The Array.map Method

The Array map method is the most useful and widely used array method among all other methods.

The Array.map method has the following syntax:

Array.map(function callback(currentValue[, index[, array]]) {
    // Return element for new_array
}[, thisArg])

The map method executes a provided function once for every element in the array and it returns a new transformed array.

Take a look at the below code:

const months = ['January', 'February', 'March', 'April'];
const transformedArray = months.map(function (month) {
  return month.toUpperCase();
});

console.log(transformedArray); // ["JANUARY", "FEBRUARY", "MARCH", "APRIL"]

Here's a Code Pen Demo.

In the above code, inside the callback function, we’re converting each element to uppercase and returning it.

The equivalent for loop code for the above example looks like this:

const months = ['January', 'February', 'March', 'April'];
const converted = [];

for(let i = 0; i < months.length; i++) {
 converted.push(months[i].toUpperCase());
};

console.log(converted); // ["JANUARY", "FEBRUARY", "MARCH", "APRIL"]

Here's a Code Pen Demo.

Using map helps to avoid creating a separate converted array beforehand for storing the converted elements. So it saves memory space and also the code looks much cleaner using array map, like this:

const months = ['January', 'February', 'March', 'April'];

console.log(months.map(function (month) {
  return month.toUpperCase();
})); // ["JANUARY", "FEBRUARY", "MARCH", "APRIL"]

Here's a Code Pen Demo.

Note that the map method returns a new array that is of the exact same length as the original array.

The difference between the forEach and map methods is that forEach is only used for looping and does not return anything back. On the other hand, the map method returns a new array that is of the exact same length as the original array.

Also, note that map does not change the original array but returns a new array.

Take a look at the below code:

const users = [
  {
    first_name: 'Mike',
    last_name: 'Sheridan'
  },
  {
    first_name: 'Tim',
    last_name: 'Lee'
  },
  {
    first_name: 'John',
    last_name: 'Carte'
  }
];

const usersList = users.map(function (user) {
  return user.first_name + ' ' + user.last_name;
});

console.log(usersList); // ["Mike Sheridan", "Tim Lee", "John Carte"]

Here's a Code Pen Demo.

Here, by using the array of objects and map methods, we're easily generating a single array with first and last name concatenated.

In the above code, we're using the + operator to concatenate two values. But it's much more common to use ES6 template literal syntax as shown below:

const users = [
  {
    first_name: 'Mike',
    last_name: 'Sheridan'
  },
  {
    first_name: 'Tim',
    last_name: 'Lee'
  },
  {
    first_name: 'John',
    last_name: 'Carte'
  }
];

const usersList = users.map(function (user) {
  return `${user.first_name} ${user.last_name}`;
});

console.log(usersList); // ["Mike Sheridan", "Tim Lee", "John Carte"]

Here's a Code Pen Demo.

The array map method is also useful, if you want to extract only specific data from the array like this:

const users = [
  {
    first_name: 'Mike',
    last_name: 'Sheridan',
    age: 30
  },
  {
    first_name: 'Tim',
    last_name: 'Lee',
    age: 45
  },
  {
    first_name: 'John',
    last_name: 'Carte',
    age: 25
  }
];

const surnames = users.map(function (user) {
  return user.last_name;
});

console.log(surnames); // ["Sheridan", "Lee", "Carte"]

Here's a Code Pen Demo.

In the above code, we're extracting only the last names of each user and storing them in an array.

We can even use map to generate an array with dynamic content as shown below:

const users = [
  {
    first_name: 'Mike',
    location: 'London'
  },
  {
    first_name: 'Tim',
    location: 'US'
  },
  {
    first_name: 'John',
    location: 'Australia'
  }
];

const usersList = users.map(function (user) {
  return `${user.first_name} lives in ${user.location}`;
});

console.log(usersList); // ["Mike lives in London", "Tim lives in US", "John lives in Australia"]

Here's a Code Pen Demo.

Note that in the above code, we're not changing the original users array. We're creating a new array with dynamic content because map always returns a new array.

Advantages of using the map method

It helps quickly generate a new array without changing the original array
It helps generate an array with dynamic content based on each element
It allows us to quickly extract any element of the array
It generates an array with the exact same length as the original array

Browser Support:

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

The Array.find Method

The Array.find method has the following syntax:

Array.find(callback(element[, index[, array]])[, thisArg])

The find method returns the value of the first element in the array that satisfies the provided test condition.

The find method takes a callback function as the first argument and executes the callback function for every element of the array. Each array element value is passed as the first parameter to the callback function.

Suppose, we have a list of employees like this:

const employees = [
 { name: "David Carlson", age: 30 },
 { name: "John Cena", age: 34 },
 { name: "Mike Sheridan", age: 25 },
 { name: "John Carte", age: 50 }
];

and we want to get the record for the employee whose name is John. In this case, we can use the find method as shown below:

const employee = employees.find(function (employee) {
  return employee.name.indexOf('John') > -1;
});

console.log(employee); // { name: "John Cena", age: 34 }

Here's a Code Pen Demo.

Even though there is "John Carte" in the list, the find method will stop when it finds the first match. So it will not return the object with the name "John Carte".

The equivalent for loop code for the above example looks like this:

const employees = [
 { name: "David Carlson", age: 30 },
 { name: "John Cena", age: 34 },
 { name: "Mike Sheridan", age: 25 },
 { name: "John Carte", age: 50 }
];

let user;

for(let i = 0; i < employees.length; i++) {
  if(employees[i].name.indexOf('John') > -1) {
    user = employees[i];
    break;
  }
}

console.log(user); // { name: "John Cena", age: 34 }

Here's a Code Pen Demo.

As you can see, using normal for loop makes the code much larger and harder to understand. But using the find method, we can write the same code in an easy to understand way.

Advantages of using the find method

It allows us to quickly find any element without writing a lot of code
It stops looping as soon as it finds a match so there is no need for an extra break statement

Browser Support:

All modern browsers except Internet Explorer (IE)
Microsoft Edge version 12 and above

The Array.findIndex Method

The Array.findIndex method has the following syntax:

Array.findIndex(callback(element[, index[, array]])[, thisArg])

The findIndex method returns the index of the first element in the array that satisfies the provided test condition. Otherwise, it returns -1, indicating that no element passed the test.

const employees = [
  { name: 'David Carlson', age: 30 },
  { name: 'John Cena', age: 34 },
  { name: 'Mike Sheridan', age: 25 },
  { name: 'John Carte', age: 50 }
];

const index = employees.findIndex(function (employee) {
  return employee.name.indexOf('John') > -1;
});

console.log(index); // 1

Here's a Code Pen Demo.

Here we get the output as 1 which is the index of the first object with the name John. Note that the index starts with zero.

The equivalent for loop code for the above example looks like this:

const employees = [
  { name: 'David Carlson', age: 30 },
  { name: 'John Cena', age: 34 },
  { name: 'Mike Sheridan', age: 25 },
  { name: 'John Carte', age: 50 }
];

let index = -1;

for(let i = 0; i < employees.length; i++) {
  if(employees[i].name.indexOf('John') > -1) {
    index = i;
    break;
  }
}

console.log(index); // 1

Here's a Code Pen Demo.

Advantages of using the findIndex method

It allows us to quickly find the index of an element without writing a lot of code
It stops looping as soon as it finds a match so there is no need for an extra break statement
We can find the index using the array find method also, but using findIndex makes it easy and avoids creating extra variables to store the index

Browser Support:

All modern browsers except Internet Explorer (IE)
Microsoft Edge version 12 and above

The Array.filter Method

The Array.filter method has the following syntax:

Array.filter(callback(element[, index[, array]])[, thisArg])

The filter method returns a new array with all the elements that satisfy the provided test condition.

The filter method takes a callback function as the first argument and executes the callback function for every element of the array. Each array element value is passed as the first parameter to the callback function.

const employees = [
  { name: 'David Carlson', age: 30 },
  { name: 'John Cena', age: 34 },
  { name: 'Mike Sheridan', age: 25 },
  { name: 'John Carte', age: 50 }
];

const employee = employees.filter(function (employee) {
  return employee.name.indexOf('John') > -1;
});

console.log(employee); // [ { name: "John Cena", age: 34 }, { name: "John Carte", age: 50 }]

Here's a Code Pen Demo.

As can be seen in the above code, using filter helps to find all the elements from the array that match the specified test condition.

So using filter does not stop when it finds a particular match but keeps checking for other elements in the array that match the condition. Then it returns all the matching elements from the array.

The main difference between find and filter is that find only returns the first matching element of the array, but using filter returns all the matching elements from the array.

Note that the filter method always returns an array. If no element passes the test condition, an empty array will be returned.

The equivalent for loop code for the above example looks like this:

const employees = [
  { name: 'David Carlson', age: 30 },
  { name: 'John Cena', age: 34 },
  { name: 'Mike Sheridan', age: 25 },
  { name: 'John Carte', age: 50 }
];

let filtered = [];

for(let i = 0; i < employees.length; i++) {
 if(employees[i].name.indexOf('John') > -1) {
   filtered.push(employees[i]);
 }
}

console.log(filtered); // [ { name: "John Cena", age: 34 }, { name: "John Carte", age: 50 }]

Here's a Code Pen Demo.

Advantages of using the filter method

It allows us to quickly find all the matching elements from the array
It always returns an array even if there is no match, so it avoids writing extra if conditions
It avoids the need of creating an extra variable to store the filtered elements

Browser Support:

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

The Array.every Method

The Array.every method has the following syntax:

Array.every(callback(element[, index[, array]])[, thisArg])

The every method tests whether all elements in the array pass the provided test conditions and returns a boolean true or false value.

Suppose we have an array of numbers and we want to check if every element of the array is a positive number. We can use the every method to achieve it.

let numbers = [10, -30, 20, 50];

let allPositive = numbers.every(function (number) {
  return number > 0;
});
console.log(allPositive); // false 

numbers = [10, 30, 20, 50];

allPositive = numbers.every(function (number) {
  return number > 0;
});
console.log(allPositive); // true

Imagine you have a registration form, and you want to check if all of the required fields are entered or not before submitting the form. You can use every method to check for each field value easily.

window.onload = function () {
  const form = document.getElementById('registration_form');
  form.addEventListener('submit', function (event) {
    event.preventDefault();
    const fields = ['first_name', 'last_name', 'email', 'city'];
    const allFieldsEntered = fields.every(function (fieldId) {
      return document.getElementById(fieldId).value.trim() !== '';
    });

    if (allFieldsEntered) {
      console.log('All the fields are entered');
      // All the field values are entered, submit the form
    } else {
      alert('Please, fill out all the field values.');
    }
  });
};

Here's a Code Pen Demo.

Here, inside the callback function of the every method, we’re checking if each field value is not empty and returning a boolean value.

In the above code, the every method returns true if, for all the elements in the fields array, the callback function returns a true value.

If the callback function returns a false value for any of the elements in the fields array, then the every method will return false as the result.

Advantage of using the `every` method

It allows us to quickly check if all the elements match certain criteria without writing a lot of code

Browser Support:

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

The Array.some Method

The Array.some method has the following syntax:

 Array.some(callback(element[, index[, array]])[, thisArg]

The some method tests whether at least one element in the array passes the test condition given by the provided function and returns a boolean true or false value.

It returns true once it finds the first match and returns false if there is no match.

Suppose we have an array of numbers and we want to check if the array contains at least one positive element. We can use the some method to achieve it.

let numbers = [-30, 40, 20, 50];

let containsPositive = numbers.some(function (number) {
  return number > 0;
});
console.log(containsPositive); // true 

numbers = [-10, -30, -20, -50];

containsPositive = numbers.every(function (number) {
  return number > 0;
});
console.log(containsPositive); // false

There are some useful scenarios for using the some method.

`Some` method example 1:

Let's say we have a list of employees and we want to check if a particular employee is present in that array or not. We also want to get the index position of that employee if the employee is found.

So instead of using the find and findIndex methods separately, we can use the some method to do both of these.

const employees = [
  { name: 'David Carlson', age: 30 },
  { name: 'John Cena', age: 34 },
  { name: 'Mike Sheridon', age: 25 },
  { name: 'John Carte', age: 50 }
];

let indexValue = -1;
const employee = employees.some(function (employee, index) {
  const isFound = employee.name.indexOf('John') > -1;
  if (isFound) {
    indexValue = index;
  }
  return isFound;
});

console.log(employee, indexValue); // true 1

Here's a Code Pen Demo.

`Some` method example 2:

The array forEach, map, and filter methods run from start to finish until all of the elements of the array are processed. There is no way of stopping of breaking out of the loop, once a particular element is found.

In such cases, we can use the array some method. The map, forEach and some method takes the same parameters in the callback function:

The first parameter is the actual value
The second parameter is the index
The third parameter is the original array

The some method stops looping through the array once it finds a particular match as can be seen in the above example 1.

Advantages of using the some method

It allows us to quickly check if some of the elements match certain criteria without writing a lot of code
It allows us to quickly break out of the loop, which was not possible with other looping methods seen above

Browser Support:

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

The Array.reduce Method

The Array.reduce method has the following syntax:

Array.reduce(callback(accumulator, currentValue[, index[, array]])[, initialValue])

The reduce method executes a reducer function (that you provide) on each element of the array, resulting in a single output value.

Note that the output of the reduce method is always a single value. It can be an object, a number, a string, an array, and so on. It depends on what you want the output of the reduce method to generate but it's always a single value.

Suppose that you want to find the sum of all the numbers in the array. You can use the reduce method for that.

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce(function(accumulator, number) {
  return accumulator + number; 
}, 0);

console.log(sum); // 15

Here's a Code Pen Demo.

The reduce method accepts a callback function that receives accumulator, number, index and array as the values. In the above code, we’re using only accumulator and number.

The accumulator will contain the initialValue to be used for the array. The initialValue decides the return type of the data returned by the reduce method.

The number is the second parameter to the callback function that will contain the array element during each iteration of the loop.

In the above code, we have provided 0 as the initialValue for the accumulator. So the first time the callback function executes, the accumulator + number will be 0 + 1 = 1 and we're returning back the value 1.

The next time the callback function runs, accumulator + number will be 1 + 2 = 3 (1 here is the previous value returned in the last iteration and 2 is the next element from the array).

Then, the next time the callback function runs, accumulator + number will be
3 + 3 = 6(the first 3 here is the previous value returned in the last iteration and the next 3 is the next element from the array) and it will continue this way until all the elements in the numbers array are not iterated.

So the accumulator will retain the value of the last operation just like a static variable.

In the above code, initialValue of 0 is not required because all the elements of the array are integers.

So the below code will also work:

const numbers = [1, 2, 3, 4, 5];

const sum = numbers.reduce(function (accumulator, number) {
  return accumulator + number;
});

console.log(sum); // 15

Here's a Code Pen Demo.

Here, the accumulator will contain the first element of the array and number will contain the next element of the array ( 1 + 2 = 3 during the first iteration and then 3 + 3 = 6 during the next iteration, and so on).

But it’s always good to specify the initialValue of accumulator as it makes it easy to understand the return type of the reduce method and get the correct type of data back.

Take a look at the below code:

const numbers = [1, 2, 3, 4, 5];

const doublesSum = numbers.reduce(function (accumulator, number) {
  return accumulator + number * 2;
}, 10);

console.log(doublesSum); // 40

Here's a Code Pen Demo.

Here, we’re multiplying each element of the array by 2. We have provided an initialValue of 10 to the accumulator so 10 will be added to the final result of the sum like this:

[1 * 2, 2 * 2, 3 * 2, 4 * 2, 5 * 2] = [2, 4, 6, 8, 10] = 30 + 10 = 40

Suppose, you have an array of objects with x and y coordinates and you want to get the sum of x coordinates. You can use the reduce method for that.

const coordinates = [
  { x: 1, y: 2 }, 
  { x: 2, y: 3 }, 
  { x: 3, y: 4 }
];

const sum = coordinates.reduce(function (accumulator, currentValue) {
    return accumulator + currentValue.x;
}, 0);

console.log(sum); // 6

Here's a Code Pen Demo.

Advantages of using the reduce method

Using reduce allows us to generate any type of simple or complex data based on the array
It remembers the previously returns data from the loop so helps us avoid creating a global variable to store the previous value

Browser Support:

All modern browsers and Internet Explorer (IE) version 9 and above
Microsoft Edge version 12 and above

Thanks for reading!

Want to learn all ES6+ features in detail including let and const, promises, various promise methods, array and object destructuring, arrow functions, async/await, import and export and a whole lot more?

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Top comments (9)

Nicolas Hervé • Feb 19 '21

There is another syntax that let you iterate over the elements of an array :

for (const element of array) {
  // do what you want
}

I like the functional programming style but sometime it is good to be able to break the loop earlier.

Yogesh Chavan • Feb 19 '21 • Edited

That's true but for...of loop is very slow compared to normal for loop and forEach loop. So it should not be used when the performance matters.

Nicolas Hervé • Feb 19 '21

It's true that for...of loop are slower, but not very slow compared to for loop. I think this cost is due to the fact that for...of loop work not only for arrays but for iterators.

Yogesh Chavan • Feb 19 '21

Yeah, maybe. Here is a detailed analysis of the performance for all of them which is worth checking out

Alain Van Hout • Feb 19 '21 • Edited

Interesting performance comparison. It's unfortunate that in the traditional for loop they did not include assigning arr[i] to a variable since that's inherently part of the other two cases.

buphmin • Feb 19 '21

I think these may be a bit off. I did a quick test using jsbench.me, with for...i as the baseline fastest: for...of 5% slower, forEach being 35% slower, and map being 45% slower. Note that run to run one may perform faster depending on what the compiler decides to optimize, which means you need to measure many runs and aggregate.

These results also make sense given the difference in how many memory allocations and references have to be made. For...i requires very little in the way of allocations, for of requires an allocation of the value variable, both forEach and map require the allocation of the function reference and the variable. If map or forEach are ever faster than for...i or for...of then it is a specific optimization on that version of the compiler and will likely fall behind once the compiler devs optimize for...i and for...of again.

Yogesh Chavan • Feb 20 '21

I agree with you, It may depend on compiler optimization. Thanks for the detailed analysis👍

Tim Fisher • Feb 24 '21

I almost never want to unconditionally visit every item in an array. forEach() takes away break, continue, and return. I almost always want to use these inside the loop. They're 10x more useful than forEach(). In fact I avoid forEach() even when I don't need break, continue, and return, just in case I need them later. I'll stick with for, but map, reduce, find, filter are all extremely useful. I wish they were in Set too.

Yogesh Chavan • Feb 24 '21

That's true. except forEach, all other methods are really useful. forEach is not used much but knowing how to use all of them is good to know.

Thanks for your point of view regarding it 🙂

DEV Community

The Most Useful JavaScript Array Methods Explained with Examples

The Array.forEach Method

Advantages of using forEach instead of a for loop

Browser Support

The Array.map Method

Advantages of using the map method

Browser Support:

The Array.find Method

Advantages of using the find method

Browser Support:

The Array.findIndex Method

Advantages of using the findIndex method

Browser Support:

The Array.filter Method

Advantages of using the filter method

Browser Support:

The Array.every Method

Advantage of using the `every` method

Browser Support:

The Array.some Method

`Some` method example 1:

`Some` method example 2:

Advantages of using the some method

Browser Support:

The Array.reduce Method

Advantages of using the reduce method

Browser Support:

Thanks for reading!

Top comments (9)

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The Array.forEach Method

Advantages of using forEach instead of a for loop

Browser Support

The Array.map Method

Advantages of using the map method

Browser Support:

The Array.find Method

Advantages of using the find method

Browser Support:

The Array.findIndex Method

Advantages of using the findIndex method

Browser Support:

The Array.filter Method

Advantages of using the filter method

Browser Support:

The Array.every Method

Advantage of using the every method

Browser Support:

The Array.some Method

Some method example 1:

Some method example 2:

Advantages of using the some method

Browser Support:

The Array.reduce Method

Advantages of using the reduce method

Browser Support:

Thanks for reading!

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Advantage of using the `every` method

`Some` method example 1:

`Some` method example 2: