JavaScript Transforming and Reordering Array Methods

map method

The map method has its syntax shown below:

array.map(func)

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 ]

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sort method

The sort method has its syntax shown below:

array.sort([func])

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
]
*/

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' ]

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
]
*/

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);

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
]
*/

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!)

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reverse method

The reverse method has its syntax shown below:

array.reverse([func])

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
]
*/

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
]
*/

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split and join methods

split method

The split method has its syntax shown below:

array.split(delit[, length])

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.
*/

};

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.
*/

};

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

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)

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

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reduce or reduceRight methods

The reduce method has its syntax shown below:

array.reduce(func)

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 )

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

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