JavaScriptのメモ

これは何？

個人的なメモです。

書き換え？

boolean

!0 === true // true 
!1 === false // true

if

let a = 100
let b = false
if( b === false ) {
  a += 100
}
console.log(a) // a = 200

↑↑↑同義↓↓↓

let a = 100
let b = false
b === false && ( a += 100 )
console.log(a) // a = 200

置換

repelace （非破壊）

let some_word = '123456789'
some_word.replace(/4567/, '')
> "12389"

console.log('some_word=[%s]', some_word)
> some_word=[123456789]

Array

破壊を非破壊処理にする

let some_array = [1, 2, 3, 4, 5, 6, 7, 8]

some_array.pop() // 破壊
some_array.slice().pop() // 非破壊

サイズ指定＆同じ数値で初期化

let N = 10
let init_num = 0
let some_array = new Array(N).fill(0)

Math.sum()

let some_array = [1, 2, 3, 4, 5, 6, 7, 8]
some_array.reduce((a, b) => a + b, 0)

Math.max()

let some_array = [1, 2, 3, 4, 5, 6, 7, 8]
let max_num = Math.max(...some_array)

数値としてsort() [破壊]

let some_array = [9, 1, 12, 13, 4, 15, 6, 7, 8]
// 破壊
some_array.sort( (a, b) => { return a - b })　// 昇順
some_array.sort( (a, b) => { return b - a })　// 降順

forEachでkey,value

let some_array = [9, 1, 12, 13, 4, 15, 6, 7, 8]
some_array.forEach( (val, key) => {
  console.log('key=[%s] val=[%i]', key, val)
})

要素分割

let some_array = [9, 1, 12, 13, 4, 15, 6, 7, 8]

some_array.slice().splice(3, 5) // index=3 から 5 要素 
> (3) [13, 7, 8]

some_array.slice().slice(5) // index=6 から最後までの要素
> (4) [15, 6, 7, 8]

重複要素の削除(distinct)

const unique = (value, index, self) => {
  return self.indexOf(value) === index
}

const ages = [26, 27, 26, 26, 28, 28, 29, 29, 30]
const uniqueAges = ages.filter(unique)

console.log(uniqueAges)
> (5) [26, 27, 28, 29, 30]

Object

ObjectのキーをArrayで取得

let some_obj = {
  a: 0,
  b: 1,
  c: 2
}
let keys = Object.keys(some_obj)

ObectをforEach

let some_obj = {
  a: 0,
  b: 1,
  c: 2
}
Object.keys(some_obj).forEach( key => {
  let val = some_obj[key]
  console.log('key=[%s] val=[%i]', key, val)
})

ObjectのValue最大

arr_idx は arr のindex。

arr_idx = []
let arr = Object.keys( obj ).map( ( key ) => {
  arr_idx.push(key)
  return obj[key]
})
let min = Math.min.apply( null, arr );
let max = Math.max.apply( null, arr );

console

%s : String
%d or %i : Number
%f : Floating points
%o : an Object
%j : JSON

Reference

• JavaScriptの配列の使い方まとめ。要素の追加,結合,取得,削除。 - Qiita