Outlook 365 – Hide Sidebar on Start in Online Calendar

Install Tampermonkey (Chrome extension) to run JavaScript code on page load.

Add a new script below, applies only for Outlook online calendar webpage, The code simply clicks the "hide sidebar button", after page loads. Uses the excellent "waitForKeyElements", helper function.

// ==UserScript==
// @name         Outlook hide left pane (brock)
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://outlook.office.com/calendar/view/workweek
// @icon         https://www.google.com/s2/favicons?domain=office.com
// @require  http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js
// @require  https://gist.github.com/raw/2625891/waitForKeyElements.js
// @grant        none
// ==/UserScript==


(function() {
    'use strict';


    /*--- waitForKeyElements():  A utility function, for Greasemonkey scripts,
    that detects and handles AJAXed content.
    Usage example:
        waitForKeyElements (
            "div.comments"
            , commentCallbackFunction
        );
        //--- Page-specific function to do what we want when the node is found.
        function commentCallbackFunction (jNode) {
            jNode.text ("This comment changed by waitForKeyElements().");
        }
    IMPORTANT: This function requires your script to have loaded jQuery.
*/
    function waitForKeyElements (
    selectorTxt,    /* Required: The jQuery selector string that
                        specifies the desired element(s).
                    */
     actionFunction, /* Required: The code to run when elements are
                        found. It is passed a jNode to the matched
                        element.
                    */
     bWaitOnce,      /* Optional: If false, will continue to scan for
                        new elements even after the first match is
                        found.
                    */
     iframeSelector  /* Optional: If set, identifies the iframe to
                        search.
                    */
    ) {
        var targetNodes, btargetsFound;

        if (typeof iframeSelector == "undefined")
            targetNodes     = $(selectorTxt);
        else
            targetNodes     = $(iframeSelector).contents ()
                .find (selectorTxt);

        if (targetNodes  &&  targetNodes.length > 0) {
            btargetsFound   = true;
            /*--- Found target node(s).  Go through each and act if they
            are new.
        */
            targetNodes.each ( function () {
                var jThis        = $(this);
                var alreadyFound = jThis.data ('alreadyFound')  ||  false;

                if (!alreadyFound) {
                    //--- Call the payload function.
                    var cancelFound     = actionFunction (jThis);
                    if (cancelFound)
                        btargetsFound   = false;
                    else
                        jThis.data ('alreadyFound', true);
                }
            } );
        }
        else {
            btargetsFound   = false;
        }

        //--- Get the timer-control variable for this selector.
        var controlObj      = waitForKeyElements.controlObj  ||  {};
        var controlKey      = selectorTxt.replace (/[^\w]/g, "_");
        var timeControl     = controlObj [controlKey];

        //--- Now set or clear the timer as appropriate.
        if (btargetsFound  &&  bWaitOnce  &&  timeControl) {
            //--- The only condition where we need to clear the timer.
            clearInterval (timeControl);
            delete controlObj [controlKey]
        }
        else {
            //--- Set a timer, if needed.
            if ( ! timeControl) {
                timeControl = setInterval ( function () {
                    waitForKeyElements (    selectorTxt,
                                        actionFunction,
                                        bWaitOnce,
                                        iframeSelector
                                       );
                },
                                           300
                                          );
                controlObj [controlKey] = timeControl;
            }
        }
        waitForKeyElements.controlObj   = controlObj;
    }

    // Your code here...
    waitForKeyElements ("#app > div > div._3KAPMPOz8KsW24ooeUflK2 > div.VDYFWHrYSe2x577N5ql-i > div.O0VoCp2Pdm6eUS4HjTWpF > div > div > div._1A05dGbwExRKqtFqnr0Z1Q > div.WbpJjWVi6dufpg4aptZk2 > button", actionFunction, true);
    // make sure to pass in true as the third argument to waitForKeyElements if you only want your code to run once – Starwarswii
    // javascript - Userscript to wait for page to load before executing code techniques? - Stack Overflow
    // https://stackoverflow.com/questions/12897446/userscript-to-wait-for-page-to-load-before-executing-code-techniques

    function actionFunction (jNode) {
        //-- DO WHAT YOU WANT TO THE TARGETED ELEMENTS HERE.
        // jNode.css ("background", "yellow"); // example
        jNode.click();
    }

})();