实现JavaScript forEach

    function forEach(list, callback){
        for(var n = ; n <list.length; n++){
            callback.call(list[n], n);
        }
    }
    var arr = [,,,,,,,]
    forEach(arr, function(index){
        if(this == arr[index]){
            console.log("Got the expected value of " + arr[index]);
        }
    })

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