Javascript built-in data structure - Array, part 3/3

// Filter array values with given condition, method filter
const arr18 = ['a', 'd', 'c', ['k', 6], ['g', 0], ['h', 1]]
console.log(arr18.filter((el) => typeof el === 'string')) // only return string element
// [ 'a', 'd', 'c' ]
console.log(arr18.filter((el) => Array.isArray(el))) // only return array element
// [ [ 'k', 6 ], [ 'g', 0 ], [ 'h', 1 ] ]
console.log(arr18.filter((el) => el[1] > 0)) // only return array element has second element greater than 0
// [ [ 'k', 6 ], [ 'h', 1 ] ]

// with additional bar variable
const bar = 2
const filtered = arr18.filter((el) => {
  return Array.isArray(el) && el[1] >= bar // only return array element has second element greater than `bar`
}, bar) 
console.log(filtered) // [ [ 'k', 6 ] ]

// Determines if the array contains a value, method includes
const arr19 = [1,2,3, 'a', 'b', 'c']
console.log(arr19.includes(1)) // true
console.log(arr19.includes('a')) // true
console.log(arr19.includes('a', 4)) // false, start comparing from index 4
console.log(arr19.includes('d')) // false
console.log(arr19) // [ 1, 2, 3, 'a', 'b', 'c' ], original array NOT modified

// Search the first index of a value in an array, method indexOf
const arr20 = [1,2,3, 'a', 'b', 1, 'b']
console.log(arr20.indexOf(1)) // 0
console.log(arr20.indexOf('b')) // 4
console.log(arr20.indexOf('b', 5)) // 6, start search from index 5
console.log(arr20.indexOf('d')) // -1
console.log(arr20) // [ 1, 2, 3, 'a', 'b', 1, 'b' ], original array NOT modified

// Search the last index of a value in an array, method lastIndexOf
const arr21 = [1,2,3, 'a', 'b', 1, 'b']
console.log(arr21.lastIndexOf(1)) // 5
console.log(arr21.lastIndexOf('b')) // 6
console.log(arr21.lastIndexOf('b', 5)) // 4, start search from index 5
console.log(arr21.lastIndexOf('d')) // -1
console.log(arr21) // [ 1, 2, 3, 'a', 'b', 1, 'b' ], original array NOT modified

// Joins all elements of an array into a string, method join
const arr22 = [1,2,3, 'a', 'b','c']
console.log(arr22.join()) // 1,2,3,a,b,c
console.log(arr22.join('')) // 123abc
console.log(arr22.join(' _ ')) // 1 _ 2 _ 3 _ a _ b _ c

// Joins all elements of an array into a string, method join
const arr23 = [1,2,3, 'a', 'b','c']
console.log(arr23.slice()) // [ 1, 2, 3, 'a', 'b', 'c' ], same with original
console.log(arr23.slice(2)) // [ 3, 'a', 'b', 'c' ]
console.log(arr23.slice(2,4)) // [ 3, 'a' ]

// Joins all elements of an array into a string, method join
const arr24 = [1,2,3]
const extra = 1
const squared = arr24.forEach((el) => {
  console.log(el * el + extra)
}, extra)
// 2
// 5
// 10
console.log(squared) // undefined
console.log(arr24) // [ 1, 2, 3 ], original array NOT modified

// Mapping array to a new array, method map
const arr25 = [['a', 1], ['b', 2], ['c', 3]]
const year = 2020
// this mapping creates a sentence for each element with name age information
const p = arr25.map((el) => `The year is ${year} and '${el[0]}' is ${el[1]} year(s)old.`, year)
console.log(p) // 
// [
//   "The year is 2020 and 'a' is 1 year(s)old.",
//   "The year is 2020 and 'b' is 2 year(s)old.",
//   "The year is 2020 and 'c' is 3 year(s)old."
// ]
console.log(arr25) // [ [ 'a', 1 ], [ 'b', 2 ], [ 'c', 3 ] ], original array NOT modified

// Loop through all element and reduce, method reduce
const arr26 = [1,2,3]
const init = 4
// this mapping creates a sentence for each element with name age information
const r = arr26.reduce((acc, cur) => {
  return acc + cur // return value here will be `acc` for next reduceFunction call
}, 4)
console.log(r) // 10

// Check if at least one element satisifies given condition, method some
const arr27 = [1,2,3]
console.log(arr27.some((el) => el > 2)) // true
console.log(arr27.some((el) => el < 0)) // false