While learning JavaScript, I had actually missed these topics and was about to switch to React JS. Luckily, I didn't skip these topics and learnt these functions and methods as well. Hence, whoever is learning JavaScript should read this blog before learning React.🙌

1. forEach()

The forEach() method calls a function once for each element in an array, without mutating the array. It is one of the useful methods while iterating through items in an array.
forEach() takes two arguments - first being the callback function which is called for each item in an array and the second argument is optional which refers to the value of this set in the callback.

Syntax:

array.forEach(callback [ , thisArgument])

Here is an example-

const companies = [
  { name: "amazon" , start: 1971, end: 2001},
  { name: "netflix" , start: 1972, end: 2004},
  { name: "flipkart" , start: 1971, end: 2009}
  ];

companies.forEach(company => { 
    console.log(company);
});
//Object { name: "amazon", start: 1971, end: 2001 }
//Object { name: "netflix", start: 1972, end: 2004 }
//Object { name: "flipkart", start: 1971, end: 2009 }

company is a callback function here. companies.forEach(company) executes company function for every item in companies, setting the iterated item as a first argument. That's how forEach() method works.

Now if we want to iterate to items with only name property in the array companies, here is how we do it:

const companies = [
  { name: "amazon" , start: 1971, end: 2001},
  { name: "netflix" , start: 1972, end: 2004},
  { name: "flipkart" , start: 1971, end: 2009}
  ];

companies.forEach( company => { 
    console.log(company.name);
});
// "amazon"
// "netflix"
// "flipkart"

Where to use forEach() ?

forEach() is widely used to iterate array items, without breaking and having simultaneously some side-effect. Or you can go for array methods as an alternative.

2. filter()

The filter() method creates a new array with all elements that pass the test implemented by the provided function. If no elements pass the test, an empty array will be returned.

Syntax:

array.filter(function(currentValue, index, arr), thisValue)
thisValue is an optional parameter.

Lets say we want to filter out the adults below 21 yrs of age, here is an example-

const ages = [45,67,7,21,4,19,11];

const adultPersons = ages.filter( age => {
    if(age>=21) {
        return true;
    } 
});
console.log(adultPersons);
// Array [45, 67, 21]

We created a new variable adultPersons where we could store all the array items of ages which passed the test that we provided in the function.

Another example of filter() method where we want to get companies which started in 1971-

const companies = [
  { name: "amazon" , start: 1971, end: 2001},
  { name: "netflix" , start: 1972, end: 2004},
  { name: "flipkart" , start: 1971, end: 2009}
  ];

const year = companies.filter(company => { 
    if( company.start == "1971") {
    return true;
    }
});
console.log(year);
//output-->  Array [Object { name: "amazon", start: 1971, end: 2001 }, Object { name: "flipkart", start: 1971, end: 2009 }]

We got two objects from the array companies which include 1971 in start category.

3. map()

The map() method creates a new array with the results of calling a provided function for every array element. This method doesn't execute the function for the array elements without values.
The map() method calls the provided function once for each element in an array, in order.

Syntax:

array.map(function(currentValue, index, arr), thisValue)
thisValue is an optional parameter.

Here is an example of getting all company names using map()-

const companies = [
  { name: "amazon" , start: 1971, end: 2001},
  { name: "netflix" , start: 1972, end: 2004},
  { name: "flipkart" , start: 1971, end: 2009}
  ];

const compNames = companies.map(company => { 
        return company.name;
    });
console.log(compNames);
//Array ["amazon", "netflix", "flipkart"]

We got a simple array with all the company names. Easy peasy!✌

Now if we want to get the company names and end year using map() method, we can use template literals/strings too.

Here is an example-

const array = [
  { name: "amazon" , start: 1971, end: 2001, owner: "A"},
  { name: "netflix" , start: 1972, end: 2004, owner: "B"},
  { name: "flipkart" , start: 1971, end: 2009, owner: "C"}
  ];

const compNames = array.map(company =>
   `${company.name} [${company.end} - ${company.owner}]` );

console.log(compNames);
// Array ["amazon [2001 - A]", "netflix [2004 - B]", "flipkart [2009 - C]"]

Now we have got an array with each value i.e. company name along with their end year and owner.

Another example of map() where we can use Math object -

const num =[ 4 , 16 , 36];

const numSquareRoot = num.map(number => Math.sqrt(number));
console.log(numSquareRoot);
//Array [2, 4, 6]

Simple! isn't it? I hope you have learnt now how to use map() method. Practice and you'll learn more.🙌

4. sort()

The sort() method sorts the elements of an array in place and returns the sorted array. The sort order can be either alphabetic or numeric as well as ascending or descending.

Note --> the sort() method sorts the values as strings in alphabetical and ascending order.
This method works well for strings ("Ant" comes before "Bat"). However, if numbers are sorted as strings, "30" is bigger than "200", because "3" is bigger than "2".
Thus, the sort() method generates an incorrect result when sorting numbers.
A "compare function" comes useful for fixing this.

Lets say we want to sort the companies on basis of their start years. Here is an example-

const companies= [
  { name: "amazon" , start: 1971, end: 2001, owner: "A"},
  { name: "netflix" , start: 1962, end: 2004, owner: "B"},
  { name: "flipkart" , start: 1991, end: 2009, owner: "C"}
  ];

//using short method via ternary operators
const sortedCompanies = companies.sort((a,b) => (a.start > b.start ? 1 : -1));
console.log(sortedCompanies);
//Array [Object { name: "netflix", start: 1962, end: 2004, owner: "B" }, Object { name: "amazon", start: 1971, end: 2001, owner: "A" }, Object { name: "flipkart", start: 1991, end: 2009, owner: "C" }]

Here the sort() method takes a function that takes two values a and b and compares them on the basis of their start date.
Note -> a denotes company one and b denotes company two.

Another example of sort() -

const num = [34,4,67,12,1];
const sortNum = num.sort();
console.log(sortNum);
//output->  Array [1, 12, 34, 4, 67]

//for ascending order
const numAscend = num.sort((a,b) => a-b);
console.log(numAscend);
//output->  Array [1, 4, 12, 34, 67]

// for descending order
const numDescend = num.sort((a,b)=> b-a);
console.log(numDescend);
//output->  Array [67, 34, 12, 4, 1]

5. reduce()

The reduce() method reduces the array to a single output value. This method executes a provided function for each value of the array (from left-to-right). The return value of the function is stored in an accumulator(total).

Syntax:

array.reduce(function(total, currentValue, currentIndex, arr), initialValue)

Note: reduce() does not execute the function for array elements without values.

Here is an example of finding sum of numbers in an array using reduce method -

const num = [45,5,2,10];

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

console.log(sum);
// 62

Here, we pass two arguments total and number as arguments in the function. total denotes previously returned value of the function and number denotes the value of current element. We have also passed second parameter 0 which means its going to start from 0.
You can use arrow functions too. I didn't use arrow functions so that beginners could understand what's happening with the function.

Another example of reduce method to get total years for all companies-

const companies= [
  { name: "amazon" , start: 1970, end: 2000, owner: "A"},
  { name: "netflix" , start: 1960, end: 2000, owner: "B"},
  { name: "flipkart" , start: 1990, end: 2010, owner: "C"}
  ];

const totalYears = companies.reduce((total, company) => 
    total + (company.end - company.start),0);

console.log(totalYears);
//90

I hope you find this post useful and informative. Share your feedback on comments section. If you have queries, reach out to me on linkedin , instagram, github, twitter. 😀