static: add rest.js to handle PATCH requests & respective responses

Add `rest.js` file to have a utilities JavaScript module that can be
reused by any Patchwork JS files that make requests to the REST API. The
comments for each function follow the Google JS Style guide [1] which is
something that would be nice to have for better documented frontend code,
especially for JS modules that export functions like rest.js. In
particular, this patch provides the following function:

 - `updateProperty`: make PATCH requests that partially update the
   fields of an object given it's REST API endpoint specified by the
   caller. Also, the caller can specify the field(s) to modify and the
   associated content for update messages in the case of both failed
   successful requests that render to the current webpage. The caller
   receives whether the request was successful or not.

The `rest.js` module can be further expanded to support and provide
functions that allow for other requests (e.g. GET, POST, PUT) to the
REST API.

Also, add functions that handle update & error messages for these PATCH
requests that match the Django messages framework format and form error
styling. These functions are internal to the module and aren't exposed
outside of the `rest.js` file.

Error and accompanying failed update messages are replaced by successful
update messages and vice versa. Consecutive successful update messages
add to a counter of updated objects. Consecutive error messages stack up.

Signed-off-by: Raxel Gutierrez <raxel@google.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>

Author: Raxel Gutierrez

htdocs/README.rst

@@ -122,6 +122,11 @@ js
   :GitHub: https://github.com/js-cookie/js-cookie/
   :Version: 3.0.0
 
+``rest.js.``
+  Utility module for REST API requests to be used by other Patchwork JS files.
+
+  Part of Patchwork.
+
 ``selectize.min.js``
   Selectize is the hybrid of a ``textbox`` and ``<select>`` box. It's jQuery
   based and it has autocomplete and native-feeling keyboard navigation; useful

htdocs/js/rest.js

@@ -0,0 +1,118 @@
+/**
+ * Sends PATCH requests to update objects' properties using the REST API.
+ * @param {string} url Path to the REST API endpoint.
+ * @param {{field: string, value: string}} data
+ *     field: Name of the property field to update.
+ *     value: Value to update the property field to.
+ * @param {{error: string, success: string}} updateMessage
+ *     error: Message when object update failed due to errors.
+ *     success: Message when object update successful.
+ * @return {boolean} Whether the request was successful.
+ */
+async function updateProperty(url, data, updateMessage) {
+    const request = new Request(url, {
+        method: 'PATCH',
+        mode: "same-origin",
+        headers: {
+            // Get csrftoken using 'js-cookie' module
+            "X-CSRFToken": Cookies.get("csrftoken"),
+            "Content-Type": "application/json",
+        },
+        body: JSON.stringify(data),
+    });
+
+    return await fetch(request)
+        .then(response => {
+            let message = updateMessage.success;
+            if (!response.ok) {
+                response.json().then(responseObject => {
+                    // Add error messages from response body to page
+                    // which can be an array of errors for a given key
+                    for (const [key, value] of Object.entries(responseObject)) {
+                        if (Array.isArray(value)) {
+                            for (const error of value) {
+                                handleErrorMessage(`${key} : ${error}`);
+                            }
+                        } else {
+                            handleErrorMessage(`${key} : ${value}`);
+                        }
+                    }
+                });
+                // Update message to be unsuccessful
+                message = updateMessage.error;
+            }
+            handleUpdateMessage(message, response.ok);
+            return response.ok
+        }).catch(error => {
+            handleErrorMessage(error);
+            return false
+        });
+}
+
+/**
+ * Populates update messages for API REST requests.
+ * @param {string} message Text for update message.
+ * @param {boolean} success Whether the request was successful.
+ */
+function handleUpdateMessage(message, success) {
+    // Replace error and failure update messages with success update message
+    const errorContainer = document.getElementById("errors");
+    let messages = document.getElementsByClassName("messages")[0];
+    if (success && errorContainer.firstChild != null) {
+        messages.replaceChildren();
+        errorContainer.replaceChildren();
+    } else if (!success) {
+        messages.replaceChildren();
+    }
+
+    // Increment counter of consecutive success update messages
+    if (messages.firstChild != null) {
+        const currentMessageCount = messages.firstChild.textContent.match('^([0-9]+)');
+        // Number matched in message
+        if (currentMessageCount != null) {
+            const newMessageCount = parseInt(currentMessageCount) + 1;
+            message = newMessageCount + message.slice(1);
+        } else {
+            // No number matched in message
+            message = "1" + message.slice(1);
+        }
+    }
+
+    // Create new message element and add to list
+    const messageElem = document.createElement("li");
+    messageElem.setAttribute("class", "message");
+    if (success) {
+        messageElem.classList.add("success");
+    } else {
+        messageElem.classList.add("error");
+    }
+    messageElem.textContent = message;
+    messages.replaceChildren(...[messageElem]);
+}
+
+/**
+ * Populates error messages for API REST requests.
+ * @param {string} message Text for error message.
+ */
+function handleErrorMessage(message) {
+    let errorContainer = document.getElementById("errors");
+    let errorHeader = document.getElementById("errors-header");
+    let errorList = document.getElementsByClassName("error-list")[0];
+
+    // Create errors list and error header if container contents removed
+    if (errorList == null) {
+        errorHeader = document.createElement("p");
+        errorList = document.createElement("ul");
+        errorHeader.setAttribute("id", "errors-header")
+        errorHeader.textContent = "The following errors were encountered while making updates:";
+        errorList.setAttribute("class", "error-list");
+        errorContainer.appendChild(errorHeader);
+        errorContainer.appendChild(errorList);
+    }
+
+    const error = document.createElement("li");
+    error.textContent = message;
+    errorList.appendChild(error);
+}
+
+export { updateProperty };