Change SharePoint Welcome Page with CSCOM

changeSPWelcomePage.js

SP.SOD.executeFunc('SP.js', 'SP.ClientContext', function() {

    var clientContext = new SP.ClientContext.get_current();
    var web = clientContext.get_web();
    var rootFolder = web.get_rootFolder();

    clientContext.load(rootFolder);
    clientContext.executeQueryAsync(function() {
        console.log("WebWelcome Page is: " + rootFolder.get_welcomePage());

        rootFolder.set_welcomePage("SitePages/HomeM.aspx");
        rootFolder.update();
    });
});