JavaScript 有用的代码片段和 trick(5)

// 对象
function isObject(value) {
 return Object.prototype.toString.call(value).slice(8, -1) === 'Object'';
}
// 数组
function isArray(value) {
 return Object.prototype.toString.call(value).slice(8, -1) === 'Array';
}
// 函数
function isFunction(value) {
 return Object.prototype.toString.call(value).slice(8, -1) === 'Function';
}

toString(); // Uncaught SyntaxError: Invalid or unexpected token
..toString(); // 第二个点号可以正常解析
 .toString(); // 注意点号前面的空格
(2).toString(); // 2先被计算

var suffix = ' name';
var person = {
 ['first' + suffix]: 'Nicholas',
 ['last' + suffix]: 'Zakas'
}
person['first name']; // "Nicholas"
person['last name']; // "Zakas"

// v: 值，p: 精度
function (v, p) {
 p = Math.pow(10, p >>> 31 ? 0 : p | 0)
 v *= p;
 return (v + 0.5 + (v >> 31) | 0) / p
}
round(123.45353, 2); // 123.45

function download(url) {
 var isChrome = navigator.userAgent.toLowerCase().indexOf('chrome') > -1;
 var isSafari = navigator.userAgent.toLowerCase().indexOf('safari') > -1;
 if (isChrome || isSafari) {
  var link = document.createElement('a');
  link.href = url;
  if (link.download !== undefined) {
   var fileName = url.substring(url.lastIndexOf('/') + 1, url.length);
   link.download = fileName;
  }
  if (document.createEvent) {
   var e = document.createEvent('MouseEvents');
   e.initEvent('click', true, true);
   link.dispatchEvent(e);
   return true;
  }
 }
 if (url.indexOf('?') === -1) {
  url += '?download';
 }
 window.open(url, '_self');
 return true;
}

function uuid() {
 var d = new Date().getTime();
 var uuid = 'xxxxxxxxxxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
  var r = (d + Math.random() * 16) % 16 | 0;
  d = Math.floor(d / 16);
  return (c == 'x' ? r : (r & 0x3 | 0x8)).toString(16);
 });
 return uuid;
};
uuid(); // "33f7f26656cb-499b-b73e-89a921a59ba6"

function isEqual(n1, n2, epsilon) {
 epsilon = epsilon == undefined ? 10 : epsilon; // 默认精度为10
 return n1.toFixed(epsilon) === n2.toFixed(epsilon);
}
0.1 + 0.2; // 0.30000000000000004
isEqual(0.1 + 0.2, 0.3); // true
0.7 + 0.1 + 99.1 + 0.1; // 99.99999999999999
isEqual(0.7 + 0.1 + 99.1 + 0.1, 100); // true