All of these array methods do not mutate the original array so we don’t wory about changing the original arrays.

Filter

Include all of the items less than $100 in a new array

const filtereditems = items.filter(item => {
return item.price <= 100
})

Map

Do something to every item in an array.

const itemNames = items.map(items => {
  return item.price
})

Find

“Find” a single item in an array. Returns an item for the very first one where it’s true.

const itemNames = items.find(item => {
  return item.name === "Album"
})

forEach

Does not return anything. Works very similar to a “for loop.”

items.forEach(item => {
  console.log(item.name)
})

some

A bit different than most others in that it returns “true” or “false” instead of a brand new array.

const hasInexpensiveItems = items.some(items => {
  return item.price <= 100
})

console.log(hasInexpensiveItems)  // result true

every

Similar to the “.some()” method but instead of checking for a single item checks to see if every item meets a condition an returns “true” or “false.”

const overTenBucks = items.every(item => {
  return item.price > 10
})

// expected result is false

reduce

Different from the other methods in that it is actually doing an operation on the array and returning a combination of all those different operations. good for addition. Easier than a “for” loop. Takes a second argument.

const total  = items.reduce((currentTotal, item) => {
  return item.price + currentTotal
}, 0)

Note that the zero signifies our starting point and “currentTotal” is the running total.

The “.reduce()” method runs a function on every single item of the array.

includes

Doesn’t actually take a function it just takes a single argument and checks to see if it is inside, returns true or false. Simpler than having to do a complex “.find()” method for example when you have a very simple array.

const items = [4, 23, 6, 7, 12]

const includesTwo = items.includes(23)

// expected result is "true"

Sort

Self explanatory on desired effect. Takes two values and returns 1 or -1 to bubble throughout the array.

// sort companies by the start year from the earliest to the latest

unsortedCompanies = []


const sortedCompanies = unsortedCompanies.sort((company1, company2) => {
  if (company1.start > company2.start) {
    return 1
  } else {
    return -1
  }
})

Note: will often see “a” and “b” versus “company1” and “company2.”

Note: Also common to use a “ternary operator” in the function block.

Note: when it comes to numbers, in the case of non-leading-zeroes, you may need to use “a – b” or “b – a” syntax.

const sortAges = ages.sort(a, b => a - b)

Array.from()

Creates a new shallow copied array from an array-like or iterable object.

// convert string of numbers to an array of numbers

const str = "1234567"

const res = Array.from(str, mapFn)

function mapFn(x) {
  return Number(x)
}

// expected results [1, 2, 3, 4, 5, 6, 7]

Refactor this to an arrow function. Another use case if you wan to remove all the duplicated values in an array.

const numbers = [1, 1, 2, 3, 8, 8, 16, 32, 64]

const res = Array.from(new Set(numbers))

//expected result [1, 2, 3, 8, 16, 32, 64]

Note: a “set” in JavaScript is an object that lets you store unique values of any type.