2019-3-20

background: url(http://i.imgur.com/b9r5sEL.jpg) bottom center;
background-size: [ <length> | <percentage> | auto ]{1,2} | cover | contain;

justify-content：!flex-start | flex-end | center | space-between | space-around

应用于父级元素

align-items：flex-start | flex-end | center | baseline | ! stretch

a b c padding 不一样 应用于父级元素

text-align：! start | end | left | right | center | justify | match-parent | justify-all
/* 适用于： 块容器 */

function playSound(e) {<br />    const audio = document.querySelector(`audio[data-key="${e.keyCode}"]`)<br />    const key = document.querySelector(`div[data-key="${e.keyCode}"]`)<br />    if (!audio) return;<br />    key.classList.add('playing');<br />    audio.currentTime = 0;<br />    audio.play()<br />  }

function removeTransition(e) {
if (e.propertyName !== ‘transform’) return;
e.target.classList.remove(‘playing’);
}

//这里顺序是重要的！！
const keys = Array.from(document.querySelectorAll(‘.key’));
keys.forEach((key) => {key.addEventListener(‘transitionend’, removeTransition)});window.addEventListener(‘keydown’, playSound)

// parsing:
parseInt("20px");       // 20
parseInt("10100", 2);   // 20
parseInt("2e1");        // 2
// type conversion
Number("20px");       // NaN
Number("2e1");        // 20, exponential notation

console.log(Math.sign(3));
// expected output: 1
console.log(Math.sign(-3));
// expected output: -1
console.log(Math.sign(0));
// expected output: 0
console.log(Math.sign('-3'));
// expected output: -1

function maxChar(str) {
  const charMap = {};
  for (let char of str) {
    charMap[char] = charMap[char] + 1 || 1;
  }
  return charMap;
}