使用JavaScript提交表单

Question

我尝试使用JavaScript提交表单，我的测试已经完成。我一直收到一条JavaScript错误Uncaught TypeError: formElement.submit is not a function。

这是我添加事件监听器的代码：

window.onload = function () 
{ 
    //Event Listeners
    var messageForm = document.getElementById("messageForm");
    messageForm.addEventListener("submit",handleSubmit);
}

这是我的功能，执行formElement.submit();时发生错误。这是我的全部功能：

function handleSubmit(ev) {
    //Prevent default form submittion
    ev.preventDefault();

    //Validate the form
    var isValid = validation(this), 
                error, 
                formElement = this,
                submitButton = this.querySelector("input[type=submit]");

    //Check if validation passes
    if (isValid == true) {
        //Check if a file has been selected
        var file = this.querySelector("input[type=file]");

        if (file != "") {
            //Generate a new form
            var f = document.createElement("form");
            f.setAttribute("method", "POST");
            f.setAttribute("enctype", "multipart/form-data");

            //Create FormData Object
            var formData = new FormData(f);

            //Append file
            formData.append("image", file.files[0], "image.jpg");
            formData.append("source", "message");

            //Uploaded
            var xhr = new XMLHttpRequest();
            xhr.open("POST", "php/uploadImage.php", true);
            xhr.onload = function(e) {
                if (xhr.status == 200) {
                    //Parse XML response
                    if (window.DOMParser)
                    {
                        parser=new DOMParser();
                        xmlDoc=parser.parseFromString(xhr.responseXML,"text/xml");
                    }
                    else // Internet Explorer
                    {
                        xmlDoc=new ActiveXObject("Microsoft.XMLDOM");
                        xmlDoc.async=false;
                        xmlDoc.loadXML(xhr.responseXML);
                    }

                    var imageuploaded = xmlDoc.getElementsByTagName("uploaded").nodeValue;
                    var imageerror = xmlDoc.getElementsByTagName("imageerror").nodeValue;

                    if (imageuploaded != "") {
                        formData.append("location",imageuploaded);
                        formElement.submit();
                    } else {
                        message = "Error uploading image: " + imageerror;
                    }
                } else {
                    message = "Unable to submit form! Please contact admin: " + xhr.description;
                }
            };

            //Progress
            xhr.upload.onprogress = function(e) {
                if (e.lengthComputable) {
                    var currentPercentage = Math.round(e.loaded / e.total * 100)-1;
                    submitButton.innerHTML = "UPLOAD IMAGE " + currentPercentage + "%";
                    submitButton.style.backgroundSize = (currentPercentage+1) + "% 100%";
                    if (currentPercentage==99) {
                        submitButton.innerHTML = "Processing image";
                    }
                }
            };

            //Send data
            xhr.send(formData);

        } else {
            //Submit form with no file
            this.submit();
        }
    } else {
        //Validation failed
        ev.preventDefault();

        message = "Error: Please ensure you have completed required information!";
    }
}