web面试题汇总

• css类

1、 两种以上方式实现已知或者未知宽度的垂直水平居中

// 方法1
.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100px;
    height: 100px;
    margin: -50px 0 0 -50px;
  }
}

// 2
.wraper {
  position: relative;
  .box {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
  }
}

// 3
.wraper {
  .box {
    display: flex;
    justify-content:center;
    align-items: center;
    height: 100px;
  }
}

// 4
.wraper {
  display: table;
  .box {
    display: table-cell;
    vertical-align: middle;
  }
}

• js类

1、简单实现双向数据绑定mvvm

const data = {};
const input = document.getElementById('input');
Object.defineProperty(data, 'text', {
  set(value) {
    input.value = value;
    this.value = value;
  }
});
input.onChange = function(e) {
  data.text = e.target.value;
}

2、实现Storage，使得该对象为单例，并对localStorage进行封装设置值setItem(key,value)和getItem(key)

var instance = null;
class Storage {
  static getInstance() {
    if (!instance) {
      instance = new Storage();
    }
    return instance;
  }
  setItem = (key, value) => localStorage.setItem(key, value),
  getItem = key => localStorage.getItem(key)
}