Array callback methods

Arrays come with many built-in methods that accept callback functions.

For Each

accepts a callback function. Calls the function once per element in the array.

const num = [1,2,3,4,5,6];
num.forEach(function(n){ // n parameter represents one element at a time
    console.log(n)
})

We can also add a second parameter to our callback to the function here if we want to use the index.

num.forEach(function(e,i){
    console.log(i,e);
})

Map Method

creates a new array with the results of calling a callback on every element in the array

const texts = ['fer','rrer','rer','erre'];
const caps = texts.map(function(t){
    return t.toUpperCase();
})

Arrow Functions

Syntactically compact alternative to a regular function expression.

const square = (x)=>{
    return x*x;
}

Implicit return

all these functions do the same thing:

const isEven = function(num){
    return num%2===0;
}
const isEven = (num)=>{
    return num%2===0;
}
const isEven = num =>{
    return num%2===0;
}
const isEven = num =>{ // implicit return
    num%2===0
} 
const isEven = num=> num%2===0;// one-liner implicit return

Find method

returns the value of the first element in the array that satisfies the provided testing function.

let shoppingList = [
    "Veggies",
    "Milk",
    "Notebooks"
]
let item = shoppingList.find(item=>{
    return item.includes("Milk");
})

Filter method

creates a new array with all elements that pass the test implemented by the provided function.

const numbers = [12,334,542,3,34,54,5,45,3,4,523,6,3]
const evens = numbers.filter(n=>{
    return n%2===0;
})

Every and Some method

Every-

tests whether all elements in the array pass the provided function It returns a boolean value.

const words = ['dog','dog','dig','dag','bag'];
words.every(word=>{
    return word.length === 3;
}) // true

Some -

Similar to every, but returns true if any of the array elements pass the test function.

words.some(word=>{
    return word.length >4;
})

Sort

arr.sort(compareFunc(a,b)))

If compareFunc(a,b) returns less than 0 -> Sort a before b

If compareFunc(a,b) returns 0 -> leave a and b unchanged with respect to each other

If compareFunc(a,b) returns greater than 0 -> Sort b before a

const prices = [122,4542,453,5248,78709,3435];
prices.sort(); // it's weird converts into strings then sort

prices.sort((a,b)=> a-b);

prices.sort((a,b)=>b-a);

Reduce Method

executes a reducer function on each element of the array, resulting in a single value.

Summing an array

const arr = [3,5,7,9,11];
arr.reduce((accumulator, currentValue)=>{
    return accumulator+currentValue;
})

Callback	Accumulator	Currentvalue	return value
first call	3	5	8
second call	8	7	15
third call	15	9	24
fourth call	24	11	35

it's not always about summing or multiplying or accumulating data in one number. It could be finding the maximum value in an entire array.

Initial value

when you use reduce you can actually pass in an initial starting value. So after your callback the format would be something dot reduce.

[12,23,5,6].reduce((acc,curr)=>{
    return acc+curr;
},100)

Tallying

we can tally up results from an array we can group different values in an array using an object.

const vote = ['y','y','n','y','y','y','n']
const tally = vote.reduce((tally, vote)=>{
    tally[vote] = (tally[vote] || 0)+1
    return tally;
},{})
// {}- initial object

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