// Higher order function returning an other function
const higherOrder = n => {
@ -85,7 +87,7 @@ const higherOrder = n => {
console.log(higherOrder(2)(3)(10))
```
Let us see were we use call back functions.For instance the *forEach* method uses call back.
Let us see were we use call back functions.For instance the _forEach_ method uses call back.
```js
const numbers = [1, 2, 3, 4]
@ -102,6 +104,10 @@ const sumArray = arr => {
console.log(sumArray(numbers))
```
```sh
15
```
The above example can be simplified as follows:
```js
@ -118,7 +124,16 @@ const sumArray = arr => {
console.log(sumArray(numbers))
```
### setTime
```sh
15
```
### setting time
In JavaScript we can execute some activity on certain interval of time or we can schedule(wait) for sometime to execute some activities.
- setInterval
- setTimeout
#### setInterval
@ -126,15 +141,14 @@ In JavaScript, we use setInterval higher order function to do some activity cont
```js
// syntax
function callBack() {
function callBack() {
// code goes here
}
setInterval(callback, duration)
```
```js
function sayHello () {
function sayHello() {
console.log('Hello')
}
setInterval(sayHello, 2000) // it prints hello in every 2 seconds
@ -146,15 +160,14 @@ In JavaScript, we use setTimeout higher order function to execute some action at
```js
// syntax
function callBack() {
function callBack() {
// code goes here
}
setTimeout(callback, duration) // duration in milliseconds
```
```js
function sayHello () {
function sayHello() {
console.log('Hello')
}
setTimeout(sayHello, 2000) // it prints hello after it waits for 2 seconds.
@ -162,14 +175,14 @@ setTimeout(sayHello, 2000) // it prints hello after it waits for 2 seconds.
## Functional Programming
Instead of writing regular loop, latest version of JavaScript introduced lots of built in methods which can help us to solve complicated problems. All builtin methods take callback function. In this section, we will see *forEach*, *map*, *filter*, *reduce*, *find*, *every*, *some*, and *sort*.
Instead of writing regular loop, latest version of JavaScript introduced lots of built in methods which can help us to solve complicated problems. All builtin methods take callback function. In this section, we will see _forEach_, _map_, _filter_, _reduce_, _find_, _every_, _some_, and _sort_.
### forEach
*forEach*: Iterate an array elements. We use *forEach* only with arrays. It takes a callback function with elements, index parameter and array itself. The index and the array optional.
_forEach_: Iterate an array elements. We use _forEach_ only with arrays. It takes a callback function with elements, index parameter and array itself. The index and the array optional.
```js
arr.forEach(function(element, index, arr){
arr.forEach(function(element, index, arr){
console.log(index, element, arr)
})
// The above code can be written using arrow function
*map*: Iterate an array elements and modify the array elements. It takes a callback function with elements and index parameter and return the modified array.
_map_: Iterate an array elements and modify the array elements. It takes a callback function with elements and index parameter and return a new array.
const result = names.findIndex(name => name.length > 7)
console.log(result) // 0
const age = ages.findIndex((age) => age <20)
const age = ages.findIndex(age => age <20)
console.log(age) // 5
```
### sort
*sort*: The sort methods arranges the array elements either ascending or descending order. By default, the ***sort()*** method sorts values as strings.This works well for string array items but not for numbers. If number values are sorted as strings and it give us wrong result. Sort method modify the original array. It is recommended to copy the original document before you start sorting.
_sort_: The sort methods arranges the array elements either ascending or descending order. By default, the **_sort()_** method sorts values as strings.This works well for string array items but not for numbers. If number values are sorted as strings and it give us wrong result. Sort method modify the original array. It is recommended to copy the original data before you start using *sort* method.
🌕 You are doing great.Never give up because great things take time. You have just completed day 9 challenges and you are 9 steps a head in to your way to greatness. Now do some exercises for your brain and for your muscle.
## 💻 Exercises
```js
const countries = ['Finland', 'Sweden', 'Denmark', 'Norway', 'IceLand'];
1. Explain the difference between ***forEach, map, filter, and reduce***.
1. Explain the difference between **_forEach, map, filter, and reduce_**.
2. Define a call function before you them in forEach, map, filter or reduce.
3. Use ***forEach*** to console.log each country in the countries array.
4. Use ***forEach*** to console.log each name in the names array.
5. Use ***forEach*** to console.log each number in the numbers array.
6. Use ***map*** to create a new array by changing each country to uppercase in the countries array.
7. Use ***map*** to create an array of countries length from countries array.
8. Use ***map*** to create a new array by changing each number to square in the numbers array
9. Use ***map*** to change to each name to uppercase in the names array
10. Use ***map*** to map the products array to its corresponding prices.
11. Use ***filter*** to filter out countries containing ***land***.
12. Use ***filter*** to filter out countries having six character.
13. Use ***filter*** to filter out countries containing six letters and more in the country array.
14. Use ***filter*** to filter out country start with 'E';
15. Use ***filter*** to filter out only prices with values.
3. Use **_forEach_** to console.log each country in the countries array.
4. Use **_forEach_** to console.log each name in the names array.
5. Use **_forEach_** to console.log each number in the numbers array.
6. Use **_map_** to create a new array by changing each country to uppercase in the countries array.
7. Use **_map_** to create an array of countries length from countries array.
8. Use **_map_** to create a new array by changing each number to square in the numbers array
9. Use **_map_** to change to each name to uppercase in the names array
10. Use **_map_** to map the products array to its corresponding prices.
11. Use **_filter_** to filter out countries containing **_land_**.
12. Use **_filter_** to filter out countries having six character.
13. Use **_filter_** to filter out countries containing six letters and more in the country array.
14. Use **_filter_** to filter out country start with 'E';
15. Use **_filter_** to filter out only prices with values.
16. Find the total price of products by chaining two or more array iterators(eg. arr.map(callback).filter(callback).reduce(callback))
17. Find the sum of price of products using only reduce reduce(callback))
18. Declare a function called getStringLists which takes an array as a parameter and then returns an array only with string items.
19. Use ***reduce*** to sum all the numbers in the numbers array.
20. Use ***reduce*** to concatenate all the countries and to produce this sentence: ***Estonia, Finland, Sweden, Denmark, Norway, and IceLand are north European countries***
21. Explain the difference between ***some*** and ***every***
22. Use ***some*** to check if some names' length greater than seven in names array
23. Use ***every*** to check if all the countries contain the word land
24. Explain the difference between ***find*** and ***findIndex***.
25. Use ***find*** to find the first country containing only six letters in the countries array
26. Use ***findIndex*** to find the position of the first country containing only six letters in the countries array
27. Use ***findIndex*** to find the position of ***Norway*** if it doesn't exist in the array you will get -1.
28. Use ***findIndex*** to find the position of ***Russia*** if it doesn't exist in the array you will get -1.
29. Declare a function called ***categorizeCountries*** which returns an array of countries which have some common pattern(you find the countries array in this repository as countries.js(eg 'land', 'ia', 'island','stan')).
19. Use **_reduce_** to sum all the numbers in the numbers array.
20. Use **_reduce_** to concatenate all the countries and to produce this sentence: **_Estonia, Finland, Sweden, Denmark, Norway, and IceLand are north European countries_**
21. Explain the difference between **_some_** and **_every_**
22. Use **_some_** to check if some names' length greater than seven in names array
23. Use **_every_** to check if all the countries contain the word land
24. Explain the difference between **_find_** and **_findIndex_**.
25. Use **_find_** to find the first country containing only six letters in the countries array
26. Use **_findIndex_** to find the position of the first country containing only six letters in the countries array
27. Use **_findIndex_** to find the position of **_Norway_** if it doesn't exist in the array you will get -1.
28. Use **_findIndex_** to find the position of **_Russia_** if it doesn't exist in the array you will get -1.
29. Declare a function called **_categorizeCountries_** which returns an array of countries which have some common pattern(you find the countries array in this repository as countries.js(eg 'land', 'ia', 'island','stan')).
30. Create a function which return an array of objects, which is the letter and the number of times the letter use to start with a name of a country.
31. Declare a ***getFirstTenCountries*** function and return an array of ten countries. Use different functional programming to work on the countries.js array
32. Declare a ***getLastTenCountries*** function which which returns the last ten countries in the countries array.
33. Find out which *letter* is used many *times* as initial for a country name from the countries array (eg. Finland, Fiji, France etc)
31. Declare a **_getFirstTenCountries_** function and return an array of ten countries. Use different functional programming to work on the countries.js array
32. Declare a **_getLastTenCountries_** function which which returns the last ten countries in the countries array.
33. Find out which _letter_ is used many _times_ as initial for a country name from the countries array (eg. Finland, Fiji, France etc)
34. Use the countries information, in the data folder. Sort countries by name, by capital, by population
{country: 'United States of America', population: 323947000}
]```
console.log(mostPopulatedCountries(countries, 3))
[
{country: 'China', population: 1377422166},
{country: 'India', population: 1295210000},
{country: 'United States of America', population: 323947000}
]```
````
````
37. \*\*\* Try to develop a program which calculate measure of central tendency of a sample(mean, median, mode) and measure of variability(range, variance, standard deviation). In addition to those measures find the min, max, count, percentile, and frequency distribution of the sample. You can create an object called statistics and create all the functions which do statistical calculations as method for the statistics object. Check the output below.