# higher order functions

higher order beacuse return function

- forEach
- map
- filter
- reduce

## ForEach

forEach is not a return function
direct iterate

Syntax

// first parameter is value
[0, 1, 2, 3].forEach(function(value) {
    console.log(value)
})

// output

0
1
2
3

with index

// second parameter is index
["delhi", "goa", "up", "mp"].forEach(function(value, index) {
    console.log(index, value);
})

// output

0 "delhi"
1 "goa"
2 "up"
3 "mp"

// backend of code

function forEach(list, callback) {
    for (var i = 0; i < list.length; i++) {
        callback(
            list[i], // value
            i        // index
        )
    }
}

## map

map is return function
generate new list from given/old list

// example number
var list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var tableOf5 = list.map(function(value){
    return value * 5;
})

var tableOf6 = list.map(function(value){
    return value * 6;
})

// output
// [5, 10, 15, 20, ....., 50]
// [6, 12, 18, 24, ....., 60]

// example of string
var states = ["delhi", "goa", "up", "mp"];
var isGood = states.map(function(value) {
    return value + " is good";
})

// output
// ["delhi is good", "goa is good", "up is good", "mp is good"]

## filter

return function but condition check
generate new list from given/old list

var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
var even = numbers.filter(function(value) {
    return value % 2 == 0;
})

var odd = numbers.filter(function(value) {
    return value % 2 != 0;
})

## reduce

merge given value
generate new list from given/old list|combine string

var numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
// reduce
// first parameter function
    // first parameter is initial value
    // second parameter is list value

// second parameter initial value
var sum = numbers.reduce(function(a, value){
    return a + value
}, 0)
// output
// 55

var sum = numbers.reduce(function(a, value){
    return a + value
}, '')
// output
// 012345678910

## call by value and call by reference

### call by value

var t = 10;
var t2 = t;
console.log(t);
// 10
console.log(t2);
// 10

// Change value
t2 = 20;
console.log(t2);
// 20
console.log(t);
// 10

### call by reference

var t = [0, 1, 2];
var t2 = t;
console.log(t);
// [0, 1, 2]
console.log(t2);
// [0, 1, 2]

// Change list
t2.push(10);
console.log(t2);
// [0, 1, 2, 10]
console.log(t);
// [0, 1, 2, 10]