How to manually create multipart/form-data

Question

We can use .formData() of Body mixin to return a FormData representation of data at Chromium (Chrome) 60+ and Firefox 39+

Relevant specifications:

• 7.2 The Multipart Content-Type
• Returning Values from Forms: multipart/form-data

Errata

• Clarification of Body package data algorithm with bytes, FormData and multipart/form-data MIME type #392
• Documenting de-facto handling of multipart/form-data form field file uploads #3040

Related

• Multipart HTTP response
• How to upload files in Web Workers when FormData is not defined

How to manually create multipart/form-data using JavaScript at client and at server to serve the multipart/form-data as a response?

Answer 1

You can create multipart/form-data manually with XMLHttpRequest like this example.

function multiPost(method, url, formHash){
    var boundary = "nVenJ7H4puv"
    var body = ""
    for(var key in formHash){
        body += "--" + boundary
             + "\r\nContent-Disposition: form-data; name=" + formHash[key].name
             + "\r\nContent-type: " + formHash[key].type
             + "\r\n\r\n" + formHash[key].value + "\r\n"
    }
    body += "--" + boundary + "--\r\n"

    var xml = new XMLHttpRequest();
    xml.open(method, url)
    xml.setRequestHeader("Content-Type", "multipart/form-data; boundary=" + boundary)
    xml.setRequestHeader("Content-Length", body.length)
    xml.send(body)
}

Answer 2

You can write you own FormData polyfill, or just google it "FormData polyfill"))) And also you can use normal FormData at browsers Chrome, FireFox, Opera, Safari, IE(10+), Edge. FormData polyfill is only useful for old IE, and for workers, but for workers you better should use this - https://gist.github.com/Rob--W/8b5adedd84c0d36aba64

wikipedia

standart formdata not of body

What you need to do? You want send formdata or recieve it at js?

You can try to use my polyfill, but I have not tested it.

sample:

var data = new RawFormData();
data.append("key","value")
data.append("key", new Blob("test"), "my file.txt");
data.getOutputDeferred().then(function(formData){
    var xml = new XMLHttpRequest();
    xml.setRequestHeader("Content-Type", "multipart/form-data; boundary=" + data.getBoundry());
    xml.setRequestHeader("Content-Length", formData.length);
    xml.open(method, url);
    xml.send(formData);
});

code:

/**
* @constructor
*/
RawFormData = function () {
    this._promises = [];
    this._boundry = this.makeBoundary();
};

/**
* @return {string}
*/
RawFormData.prototype.getBoundary = function () {
    return this._boundry;
}

/**
* @return {string}
*/
RawFormData.prototype.makeBoundary = function () {
    return 'MyBoundary' + window.btoa(Math.random().toString()).substr(0, 12);
};

/**
* @param {string} name
* @param {string|number|File|Blob|boolean|null|undefined} val
* @param {string=} filename
*/
RawFormData.prototype.append = function (name, val, filename) {
    var prom = null;

    if(val instanceof File || val instanceof Blob){
        prom = this.readAsBinaryString(val).then(function(base64){
            var contentType = val.type || 'application/octet-stream';
            var result = '--' + this._boundry + '\r\n' +
                'Content-Disposition: form-data; ' +
                'name="' + name + '"; filename="' + this.encode_utf8(filename || "blob") + '"\r\n' +
                'Content-Type: ' + contentType + '\r\n\r\n' +
                base64 + '\r\n';
            return result;
        }.bind(this))
    }else{
        prom = new Promise(function(resolve){
            return '--' + this._boundry + '\r\n' +
                'Content-Disposition: form-data; ' +
                'name="' + this.encode_utf8(name) + '"\r\n\r\n' +
                this.encode_utf8(val) + '\r\n'
        }.bind(this));
    }

    this._promises.push(prom);

    return prom;
};
/**
* @return {File|Blob} blob
* @return {Promise<string>}
*/
RawFormData.prototype.readAsBinaryString = function (blob) {
        var reader = new FileReader();
        return new Promise(function(resolve,reject){
            var binStringCallback = function (e) {
                resolve(e.target.result);
            };

            var arrBufferCallback = function (e) {
                var binary = "";
                var bytes = new Uint8Array(e.target.result);
                var length = bytes.byteLength;
                for (var i = 0; i < length; i++) {
                    binary += String.fromCharCode(bytes[i]);
                }
                resolve(binary);
            };

            reader.onerror = reader.onabort = function () {
                resolve(null);
            };

            if (typeof reader.readAsBinaryString != "undefined") {
                reader.onload = binStringCallback;
                reader.readAsBinaryString(blob);
            } else {
                reader.onload = arrBufferCallback;
                reader.readAsArrayBuffer(blob);
            }
        });
};

RawFormData.prototype.encode_utf8 = function( s ){
   return unescape( encodeURIComponent( s ) );
}

RawFormData.prototype.getOutputDeferred = function () {
    return Promise.all(this._promises).then(function (rows) {
        var output = '--' + this._boundry + '\r\n';
        rows.forEach(function(row) {
            output += row;
        });
        output += '--' + this._boundry + '\r\n';
        return output;
    }.bind(this));
};