jQuery兼容浏览器IE8方法

1.前言

　　在做前端开发的时候，公司的规范要求面向客户的兼容到IE8，但是在IE8模式下经常会发现各种错误，某某方法未定义、不支持某属性或对象等，使用jQuery的时候，jQuery从2.0开始不兼容IE8，最低支持IE9，所以需要引入更低的jQuery版本来兼容。

2.IE8不支持jQuery版本解决办法

• 通过判断IE浏览器的版本来加载对应版本的jQuery

　　使用语句<!--[if IE 8]> 仅IE8可识别 <![endif]--> 可在IE8模式下进行一些兼容操作。代码如下：

<script type="text/javascript" src="<%=path%>/js/jquery-3.1.1.min.js"></script>
<!--[if IE 8]>
<script type="text/javascript" src="<%=path%>/js/jquery-1.9.1.min.js"></script>
<![endif]-->

　　这样在切换到IE8时，低版本的jQuery就会覆盖高版本的jQuery。如果在IE8下需要调整某些元素的样式的话，最好将JS代码放在页面底部（并注意是否有行内样式），不然为某些动态加载的内容设置的样式可能不会生效。

3.IE8不支持forEach解决办法

• 为不支持forEach的浏览器添加自定义forEach方法

　　代码如下：

1 if (typeof Array.prototype.forEach != 'function') {
2     Array.prototype.forEach = function (callback) {
3         for (var i = 0; i < this.length; i++) {
4             callback.apply(this, [this[i], i, this]);
5         }
6     };
7 }

如果是引入的jQuery插件，可将该段代码放在插件内容的开头即可，这样在IE8下执行forEach方法就不会报错了。 

 4.IE8不支持map解决办法

• 添加自定义forEach方法

if (!Array.prototype.map) {
    Array.prototype.map = function(callback, thisArg) {
        var T, A, k;
        if (this == null) {
            throw new TypeError(" this is null or not defined");
        }
        // 1. Let O be the result of calling ToObject passing the |this| value as the argument.
        var O = Object(this);
        // 2. Let lenValue be the result of calling the Get internal method of O with the argument "length".
        // 3. Let len be ToUint32(lenValue).
        var len = O.length >>> 0;
        // 4. If IsCallable(callback) is false, throw a TypeError exception.
        // See: http://es5.github.com/#x9.11
        if (typeof callback !== "function") {
            throw new TypeError(callback + " is not a function");
        }
        // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
        if (thisArg) {
            T = thisArg;
        }
        // 6. Let A be a new array created as if by the expression new Array(len) where Array is
        // the standard built-in constructor with that name and len is the value of len.
        A = new Array(len);
        // 7. Let k be 0
        k = 0;
        // 8. Repeat, while k < len
        while(k < len) {
            var kValue, mappedValue;
            // a. Let Pk be ToString(k).
            //   This is implicit for LHS operands of the in operator
            // b. Let kPresent be the result of calling the HasProperty internal method of O with argument Pk.
            //   This step can be combined with c
            // c. If kPresent is true, then
            if (k in O) {
                // i. Let kValue be the result of calling the Get internal method of O with argument Pk.
                kValue = O[ k ];
                // ii. Let mappedValue be the result of calling the Call internal method of callback
                // with T as the this value and argument list containing kValue, k, and O.
                mappedValue = callback.call(T, kValue, k, O);
                // iii. Call the DefineOwnProperty internal method of A with arguments
                // Pk, Property Descriptor {Value: mappedValue, : true, Enumerable: true, Configurable: true},
                // and false.
                // In browsers that support Object.defineProperty, use the following:
                // Object.defineProperty(A, Pk, { value: mappedValue, writable: true, enumerable: true, configurable: true });
                // For best browser support, use the following:
                A[ k ] = mappedValue;
            }
            // d. Increase k by 1.
            k++;
        }
        // 9. return A
        return A;
    };
}