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day 9 higher order function
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Asabeneh committed Aug 21, 2020
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17 changes: 12 additions & 5 deletions 09_Day_Higher_order_functions/09_day_higher_order_functions.md
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### map

_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.
_map_: Iterate an array elements and modify the array elements. It takes a callback function with elements, index , array parameter and return a new array.

```js
const modifiedArray = arr.map(function (element, index) {
const modifiedArray = arr.map(function (element, index, arr) {
return element
})
```
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```

```js
const countriesEndsByia = countries.filter((country) => country.includes('ia'))
const countriesEndsByia = countries.filter((country) => country.endsWith('ia'))
console.log(countriesEndsByia)
```

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### reduce

_reduce_: Reduce takes a callback function. The call back function takes accumulator and current value as a parameter and returns a single value:
_reduce_: Reduce takes a callback function. The call back function takes accumulator, current, and optional initial value as a parameter and returns a single value. It is a good practice to define an initial value for the accumulator value. If we do not specify this parameter, by default accumulator will get array `first value`. If our array is an _empty array_, then `Javascript` will throw an error.

```js
arr.reduce((acc, cur) => {
// some operations goes here before returning a value
return
}, initialValue)
```

```js
const numbers = [1, 2, 3, 4, 5]
const sum = numbers.reduce((accum, curr) => accum + curr)
const sum = numbers.reduce((acc, cur) => acc + cur, 0)

console.log(sum)
```
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