Understanding JavaScript Array Methods
JavaScript Array Methods Tutorial - Understanding The Most Useful Methods Explained with Examples
If you're a JavaScript developer, then you should be familiar with the most commonly used ES5 and ES6+ array methods.
These methods make coding in JavaScript a lot easier and also make your code look clean and easy to understand to everyone.
So in this article, we are going to explore some of the most important and widely used array methods used in JavaScript. So let's get started.
The Array.forEach Method
The Array.forEach
method has the following syntax:
Array.forEach(callbackfn: (value: element, index: number, array: []))
The
forEach
method executes a provided callback function once for every element in the array.
const months = ['January', 'February', 'March', 'April', 'May', 'June'];
months.forEach(function(month) {
console.log(month);
});
/* output
January
February
March
April
May
June
*/
Here's a Codepen Sample
Here, inside the forEach
loop callback function, each element of the array is automatically passed as the first parameter of the function.
One important 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 workDays= ['Monday', 'Tuesday', 'Wednesday', 'Thursday'];
const returnedValue = workDays.forEach(function (day) {
return day;
});
console.log('returnedValue: ', returnedValue); // undefined
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 explicitlyreturn
a value from the callbackfunction
(this means that the returned value comes asundefined
in the above example).
Advantages of using the Array.forEach Method
- 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.
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(callbackfn: (value: element, index: number, array: []))
The map method executes a provided callback function once on every element in the array and it returns a new transformed array. that contains the results.
Take a look at the below code:
const workDays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'];
const upperDays = workDays.map(function (day) {
return day.toLocaleUpperCase();
});
console.log( upperDays);
//output ['MONDAY', 'TUESDAY', 'WEDNESDAY', 'THURSDAY','FRIDAY']
Here's a Code Sample
In the above code, inside the callback function, weβre converting each element to uppercase and returning it.
Using map
method 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
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.
The array map
method is also useful if you want to extract only specific data from the array like this:
const workers = [
{
first_name: 'Mike',
last_name: 'Sheridan',
age: 30
},
{
first_name: 'Tim',
last_name: 'Lee',
age: 45
},
{
first_name: 'John',
last_name: 'Carte',
age: 25
},
{
first_name: 'Paul',
last_name: 'Victor',
age: 22
}
]
const workersAges = workers.map(function (user) {
return user.age
})
console.log(workersAges)
//output: [30, 45, 25, 22]
Here's a Code Sample
In the above code, we're extracting only the age of each worker and storing them in a new array.
Advantages of using the map method
- It helps quickly generate a new array without changing the original array
- It allows us to quickly extract any element of the array
- It generates an array with the exact same length as the original array
The Array.find Method
The Array.find
method has the following syntax:
Array.find(predicate: (value: element, index: number, array: []))
The
find
method returns the value of the first element in the array that satisfies the provided test condition. and undefined otherwise.
The find
method takes a callback function (predicate)
as the first argument and executes the callback function (predicate)
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: 32 },
{ name: "John Copper", age: 25 },
{ name: "Mike Sheridan", age: 24 },
{ name: "John Carte", age: 50 }
];
and we want to get the record for the employee whose age is less than 30. In this case, we can use the find method as shown below:
const underAgeWorker= employees.find(function (employee) {
return employee.age < 30 ;
});
console.log(underAgeWorker);
// outputs: {name: "John Copper", age: 25}
Here's a Code Sample
Even though there is "Mike Sheridan"
in the list whose age is also less than 30, the find method will stop when it finds the first match. So it will not return the first object with an age less than 30.
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
The Array.findIndex Method
The Array.findIndex
method has the following syntax:
Array.findIndex(callbackfn: (value: element, index: number, array: []))
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: 32 },
{ name: "John Copper", age: 25 },
{ name: "Mike Sheridan", age: 24 },
{ name: "John Carte", age: 50 }
];
const index = employees.findIndex(function (employee) {
return employee.name.indexOf('John') > -1;
})
console.log(index);
// outputs: 1
Here we get the output as 1 which is the index of the first object with the name John. Note that the index of an array starts with zero.
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
The Array.filter Method
The Array.filter
method has the following syntax:
Array.filter(callbackfn: (value: element, index: number, array: []))
The
filter
method takes a callback function as the first argument and executes the callback function for every element of the array and returnsa new array
with all the elements that meet the condition specified in the callback function.
So using the filter
method, it 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.
const employees = [
{ name: "David Carlson", age: 30 },
{ name: "John Cooper", age: 24 },
{ 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)
//output [ { name: "John Cooper", age: 24 }, { name: "Mike Sheridan", age: 25 } ]
Note that the
filter
method always returns an array. If no element passes the test condition, an empty array will be returned.
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
The Array.every Method
The Array.every
method has the following syntax:
Array.every(callbackfn: (value: any, index: number, array: []))
The
every
method tests whether all elements in the array pass the provided test conditions and returns a booleantrue
orfalse
value.
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 the every
method to check for each field value easily.
Here's a Code Sample
Advantage of using the every method
- It lets us to quickly check if all the elements match certain criteria without writing a lot of code
The Array.reduce Method
The Array.reduce
method has the following syntax:
Array.reduce(callbackfn: (accumulator: any, currentValue: any, Index: number, 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 ofreduce
method to generate but it's always a single value.
const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce(function(accumulator, number) {
return accumulator + number;
}, 0);
console.log(sum); // 15
The reduce
method accepts a callback function that receives accumulator
, number
, index
and array
as the values. In the above code, weβre 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 be3 + 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 number
s 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.
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
Thanks so much for reading!
That's all from me guys. hoped you've learned a little bit about the awesome JavaScript Array and its methods
If you still have any questions, don't hesitate to reach out in the comments section below!
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