Add utility helper for making form data sendable

Useful for assertions where the data you're
comparing is already in this format.

tests/javascripts/support/helpers.js

@@ -4,6 +4,7 @@ const html = require('./helpers/html.js');
 const elements = require('./helpers/elements.js');
 const rendering = require('./helpers/rendering.js');
 const forms = require('./helpers/forms.js');
+const utilities = require('./helpers/utilities.js');
 
 exports.triggerEvent = events.triggerEvent;
 exports.clickElementWithMouse = events.clickElementWithMouse;
@@ -18,3 +19,4 @@ exports.element = elements.element;
 exports.WindowMock = rendering.WindowMock;
 exports.ScreenMock = rendering.ScreenMock;
 exports.spyOnFormSubmit = forms.spyOnFormSubmit;
+exports.getFormDataFromPairs = utilities.getFormDataFromPairs;

tests/javascripts/support/helpers/utilities.js

@@ -0,0 +1,22 @@
+// general helpers, not related to the DOM and usable in different contexts
+
+// turn a list of key=value pairs (like tuples) into data that can be sent via AJAX
+// taken from https://developer.mozilla.org/en-US/docs/Learn/HTML/Forms/Sending_forms_through_JavaScript
+// but requiring an array as input rather than a hash, to preserve order of pairs
+function getFormDataFromPairs (pairs) {
+
+  const urlEncodedDataPairs = [];
+
+  pairs.forEach(pair => {
+
+    urlEncodedDataPairs.push(`${window.encodeURIComponent(pair[0])}=${window.encodeURIComponent(pair[1])}`);
+
+  });
+
+  // Combine the pairs into a single string and replace all %-encoded spaces to 
+  // the '+' character; matches the behaviour of browser form submissions.
+  return urlEncodedDataPairs.join('&').replace(/%20/g, '+');
+
+};
+
+exports.getFormDataFromPairs = getFormDataFromPairs;