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 ]
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.localeComparemethod to correctly sort letters, such asÖ.
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!)
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
]
*/
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
reduce or reduceRight methods
The reduce method has its syntax shown below:
array.reduce(func)
The
reduceandreduceRightmethods produce the same result but thereduceRightsearch 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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