DEV Community

loading...
Cover image for JavaScript Transforming and Reordering Array Methods

JavaScript Transforming and Reordering Array Methods

bello profile image Bello Updated on ・4 min read

map method

The map method has its syntax shown below:

array.map(func)
Enter fullscreen mode Exit fullscreen mode

func => func is the callback function.

See the example below:

const fruits = [ "Apple", "Orange", "Melon", "Banana", "Advocado" ];

const fruitsFunc = item => {
  return item.length;
};

console.log( fruits.map(fruitsFunc) ); // [ 5, 6, 5, 6, 8 ]
Enter fullscreen mode Exit fullscreen mode

sort method

The sort method has its syntax shown below:

array.sort([func])
Enter fullscreen mode Exit fullscreen mode

func is a callback function

See the example below:

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];

numbs.sort();

console.log(numbs); 

/*
[
   1, 12, 16, 2,
  23,  3, 31
]
*/
Enter fullscreen mode Exit fullscreen mode

The method sort() reorders the content of numbs but not as we expect because the items are sorted as strings by default (lexicographic ordering).

This means the syntax array.sort() is best for string items.

See the example below:

const fruits = [ "Apple", "Orange", "Melon", "Banana", "Advocado" ];

// the method reorders the content of arr
fruits.sort();

console.log(fruits);  // [ 'Advocado', 'Apple', 'Banana', 'Melon', 'Orange' ]
Enter fullscreen mode Exit fullscreen mode

Let's use the callback function as a parameter in the sort function

const compareNumeric = (a, b) => {
  if (a > b) return 1;
  if (a === b) return 0;
  if (a < b) return -1;
};

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];
numbs.sort(compareNumeric);

/*
[
   1,  2,  3, 12,
  16, 23, 31
]
*/
Enter fullscreen mode Exit fullscreen mode

The method is implements based on a generic sorting algorithm (mostly quicksort or Timsort).

The example above is the same as shown below:

const compareNumeric = (a, b) => {
  return a - b;
};

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];
numbs.sort(compareNumeric);
Enter fullscreen mode Exit fullscreen mode

You can also reverse the ordering shown in the example above.

const compareNumeric = (a, b) => {
  return b - a;
};

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];
numbs.sort(compareNumeric); 
/*
[
  31, 23, 16, 12,
   3,  2,  1
]
*/
Enter fullscreen mode Exit fullscreen mode

it's better to use the str.localeCompare method to correctly sort letters, such as Ö.

Below is another example:

const countries = ['Österreich', 'Andorra', 'Vietnam'];

const func1 = (a, b) => { 
    return a > b ? 1 : -1 
};

console.log( countries.sort(func1) ); 
// Andorra, Vietnam, Österreich (wrong)

const func2 = (a, b) => {
    return a.localeCompare(b);
};

console.log( countries.sort(func2) ); 
// Andorra,Österreich,Vietnam (correct!)
Enter fullscreen mode Exit fullscreen mode

reverse method

The reverse method has its syntax shown below:

array.reverse([func])
Enter fullscreen mode Exit fullscreen mode

func is a callback function

See the examples below:

const numbs = [ 1, 2, 3, 4, 5 ];
numbs.reverse();

console.log(numbs); // [ 5, 4, 3, 2, 1 ]

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];

numbs.reverse();

console.log( numbs ); 

/*
[
  31, 16, 12, 3,
   2, 23,  1
]
*/
Enter fullscreen mode Exit fullscreen mode

In the example above, the reverse method only reverses the array stored in numbs.

To properly reverse the array items we need to sort first before reversing.

See the example below:

const compareNumeric = (a, b) => {
  return a - b;
};

const numbs = [ 1, 23, 2, 3, 12, 16, 31 ];

const sortOrder = numbs.sort(compareNumeric);
const reverseOrder = sortOrder.reverse();

console.log(reverseOrder);

/*
[
  31, 23, 16, 12,
   3,  2,  1
]
*/
Enter fullscreen mode Exit fullscreen mode

split and join methods

split method

The split method has its syntax shown below:

array.split(delit[, length])
Enter fullscreen mode Exit fullscreen mode

delim => delit is the delimiter that splits the string into an array
length => The length is optional

See the example below:

const names = 'Bello, Noah, Efosa, John, Osasu';

const messages = names.split(', ');

for (let name of messages) {
  console.log( `You have an unread message from ${name}.` ); 

/*
You have an unread message from Bello.
You have an unread message from Noah.
You have an unread message from Efosa.
You have an unread message from John.
You have an unread message from Osasu.
*/

};
Enter fullscreen mode Exit fullscreen mode

The length property can also be used in the split function split(delim, length).

See the example below:

const names = 'Bello, Noah, Efosa, John, Osasu';

const messages = names.split(', ', 3);

for (let name of messages) {
  console.log( `You have an unread message from ${name}.` ); 

/*
You have an unread message from Bello.
You have an unread message from Noah.
You have an unread message from Efosa.
*/

};
Enter fullscreen mode Exit fullscreen mode

The split method can also convert a string to an array (object) of characters.

See the example below:

const str = "Bello";

const toArray = str.split('')
console.log(toArray, typeof toArray); 
// [ 'B', 'e', 'l', 'l', 'o' ] object
Enter fullscreen mode Exit fullscreen mode

join method

The join is the reverse of the split method. It creates a string of array items joined by glue between them.

See the syntax below:

array.join(glue)
Enter fullscreen mode Exit fullscreen mode

See another example below:

const str = [ 'Bello', 'Noah', 'Efosa', 'John', 'Osasu' ];

const toStr = str.join(';');
console.log( str.join(';') );
// Bello;Noah;Efosa;John;Osasu string
Enter fullscreen mode Exit fullscreen mode

reduce or reduceRight methods

The reduce method has its syntax shown below:

array.reduce(func)
Enter fullscreen mode Exit fullscreen mode

The reduce and reduceRight methods produce the same result but the reduceRight search is from right to left.

func => func is the callback function

See the example below:

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

const reducedFunc = (sum, current) => {
    return sum + current
};

const initial = 0;
const cummulateResult = numbs.reduce(reducedFunc, initial);

console.log(cummulateResult); // 15
// => 0 + (0 + 1 = 1), (0 + 1 + 2 = 3), (0 + 3 + 3 = 6), (0 + 6 + 4 = 10), (0 + 10 + 5 = 15)
// => initial + (initial + current1 = sum1), (initial + sum1 + current1 = sum2 ), ..., (initial + sumN + currentN = sumN+1 )
Enter fullscreen mode Exit fullscreen mode

The table below is a representation of the example below:

calls sum current result
first call 0 1 1
second call 1 2 3
third call 3 3 6
forth call 6 4 10
fifth call 10 5 15

Discussion (0)

pic
Editor guide