forEach兼容写法

if ( !Array.prototype.forEach ) {
    Array.prototype.forEach = function forEach( callback, thisArg ) {
        var T, k;
        if ( this == null ) {
            throw new TypeError( "this is null or not defined" );
        }
        var O = Object(this);
        var len = O.length >>> 0;
        if ( typeof callback !== "function" ) {
            throw new TypeError( callback + " is not a function" );
        }
        if ( arguments.length > 1 ) {
            T = thisArg;
        }
        k = 0;
        while( k < len ) {

            var kValue;
            if ( k in O ) {
                kValue = O[ k ];
                callback.call( T, kValue, k, O );
            }
            k++;
        }
    };
}