FormData polyfill for k6

formdata.js

/*
 * FormData polyfill for k6
 * (c) 2014 Rob Wu <rob@robwu.nl>
 * License: MIT
 *
 * This simplifies the creation of multipart/form-data requests from k6 scripts.
 * It was adapted from the original version by Rob Wu to remove references of
 * XMLHttpRequest and File related code which isn't supported in k6.
 **/
(function(exports) {
    if (exports.FormData) {
        // Don't replace FormData if it already exists
        return;
    }
    // Export variable to the global scope
    exports.FormData = FormData;

    function FormData() {
        // Force a Constructor
        if (!(this instanceof FormData)) return new FormData();
        // Generate a random boundary - This must be unique with respect to the form's contents.
        this.boundary = '------RWWorkerFormDataBoundary' + Math.random().toString(36);
        var internal_data = this.data = [];
        /**
        * Internal method.
        * @param inp String | ArrayBuffer | Uint8Array  Input
        */
        this.__append = function(inp) {
            var i = 0, len;
            if (typeof inp == 'string') {
                for (len = inp.length; i < len; ++i)
                    internal_data.push(inp.charCodeAt(i) & 0xff);
            } else if (inp && inp.byteLength) {/*If ArrayBuffer or typed array */
                if (!('byteOffset' in inp))   /* If ArrayBuffer, wrap in view */
                    inp = new Uint8Array(inp);
                for (len = inp.byteLength; i < len; ++i)
                    internal_data.push(inp[i] & 0xff);
            }
        };
    }
    /**
    * @param name     String                         Key name
    * @param value    String|typed array|ArrayBuffer Value
    * @param filename String                         Optional File name (when value is not a string).
    **/
    FormData.prototype.append = function(name, value, filename) {
        if (this.__endedMultipart) {
            // Truncate the closing boundary
            this.data.length -= this.boundary.length + 6;
            this.__endedMultipart = false;
        }
        if (arguments.length < 2) {
            throw new SyntaxError('Not enough arguments');
        }
        var part = '--' + this.boundary + '\r\n' +
                'Content-Disposition: form-data; name="' + name + '"';

        if (typeof value.byteLength == 'number') {
            // Duck-typed typed array or array buffer
            part += '; filename="'+ (filename || 'blob').replace(/"/g,'%22') +'"\r\n';
            part += 'Content-Type: application/octet-stream\r\n\r\n';
            this.__append(part);
            this.__append(value);
            part = '\r\n';
        } else {
            part += '\r\n\r\n' + value + '\r\n';
        }
        this.__append(part);
    };

    /**
    * Close finishes the multipart message and writes the trailing boundary end line to the output.
    **/
    FormData.prototype.close = function() {
        if (!this.__endedMultipart) this.__append('--' + this.boundary + '--\r\n');
        this.__endedMultipart = true;
    };
    
    /**
    * Return an ArrayBuffer with the contents of the data processed so far.
    **/
    FormData.prototype.buffer = function() {
        return new Uint8Array(this.data).buffer;
    };
})(this || self);

test-formdata.js

import http from 'k6/http';
import { FormData } from './formdata.js';

const logo1 = open('logo1.png', 'b');
const logo2 = open('logo2.png', 'b');

export default function () {
  const fd = new FormData();
  fd.append('files', logo1, 'logo1.png');
  fd.append('files', logo2, 'logo2.png');
  fd.close(); // write the trailing boundary
  const res = http.post('http://127.0.0.1:9000/postmultipart', fd.buffer(),
                       { headers: { 'Content-Type': 'multipart/form-data; boundary=' + fd.boundary }});
}