js小方法

1. 保留N位小数

   const toFixed = (n, fixed) => ~~(Math.pow(10, fixed) * n) / Math.pow(10, fixed);
   // 事例
   toFixed(25.198726354, 1);       // 25.1
   toFixed(25.198726354, 2);       // 25.19
   toFixed(25.198726354, 3);       // 25.198
   toFixed(25.198726354, 4);       // 25.1987
   toFixed(25.198726354, 5);       // 25.19872
   toFixed(25.198726354, 6);       // 25.198726

2. 检查当前是否有元素处于焦点中

   const elementIsInFocus = (el) => (el === document.activeElement);
   elementIsInFocus(anyElement)
   // 如果在焦点中返回true，如果不在焦点中返回 false

3. 获取参数平均数值

   const average = (...args) => args.reduce((a, b) => a + b) / args.length;
   average(1, 2, 3, 4);
   // 2.5

4. 获取文件后缀名

   /**
    * 获取文件后缀名
    * @param {String} filename
    */
    export function getExt(filename) {
       if (typeof filename == 'string') {
           return filename
               .split('.')
               .pop()
               .toLowerCase()
       } else {
           throw new Error('filename must be a string type')
       }
   }
   
   getExt("1.mp4") //->mp4

5. 复制内容导剪贴板

   export function copyToBoard(value) {
       const element = document.createElement('textarea')
       document.body.appendChild(element)
       element.value = value
       element.select()
       if (document.execCommand('copy')) {
           document.execCommand('copy')
           document.body.removeChild(element)
           return true
       }
       document.body.removeChild(element)
       return false
   }
   
   //如果复制成功返回true
   copyToBoard('lalallala')

6. 随机生产rbg值

   function rgb(){
       let r = Math.floor(Math.random()*256);
       let g = Math.floor(Math.random()*256);
       let b = Math.floor(Math.random()*256);
       let rgb = 'rgb('+r+','+g+','+b+')';
       return rgb;
   }

7. 随机生产16进制颜色

   function color16(){
      let r = Math.floor(Math.random()*256);
      let g = Math.floor(Math.random()*256);
      let b = Math.floor(Math.random()*256);
      let color = '#'+r.toString(16)+g.toString(16)+b.toString(16);
      return color;
   }

8. 判断是否为质数

   const mathIsPrime = n => {
     if (n === 2 || n === 3) {
       return true
     }
     if (isNaN(n) || n <= 1 || n % 1 != 0 || n % 2 == 0 || n % 3 == 0) {
       return false;
     }
     for (let x = 6; x <= Math.sqrt(n) + 1; x += 6) {
       if (n % (x - 1) == 0 || n % (x + 1) == 0) {
         return false
       }
     }
     return true
   }
   mathIsPrime(0) // true

9. 时间格式化

   const dateFormatter = (formatter, date) => {
       date = (date ? new Date(date) : new Date)
       const Y = date.getFullYear() + '',
             M = date.getMonth() + 1,
             D = date.getDate(),
             H = date.getHours(),
             m = date.getMinutes(),
             s = date.getSeconds()
       return formatter.replace(/YYYY|yyyy/g, Y)
                       .replace(/YY|yy/g, Y.substr(2, 2))
                       .replace(/MM/g, (M < 10 ? '0' : '') + M)
                       .replace(/DD/g, (D < 10 ? '0' : '') + D)
                       .replace(/HH|hh/g, (H < 10 ? '0' : '') + H)
                       .replace(/mm/g, (m < 10 ? '0' : '') + m)
                       .replace(/ss/g, (s < 10 ? '0' : '') + s)
   }
   
   dateFormatter('YYYY-MM-DD HH:mm', '1995/02/15 13:55') // 1995-02-15 13:55