Get Custom Master Page Url of a SharePoint Site using REST API

Syntax

REST API Endpoint:

https://SharePointSiteURL/_api/web?$select=CustomMasterUrl

REST API endpoint to use in Add-ins:

<appweburl>/_api/SP.AppContextSite(@target)/web?$select=CustomMasterUrl&@target=<hostweburl>

Embed Code Snippet

The following code snippet can be added to SharePoint page or in content editor web part as a script. This example is used to get the custom master page URL of a SharePoint site and this returns the site relative url.

<script type="text/javascript" src="/SiteAssets/Scripts/jquery.min.js"></script>
<script type="text/javascript">
    $.ajax(
    {
        url: _spPageContextInfo.webAbsoluteUrl + "/_api/web?$select=custommasterurl", //THE ENDPOINT
        method: "GET",
        headers:
        {
            "Accept": "application/json; odata=verbose"
        },
        success: function(data)
        {
            //RESULTS HERE!!
            console.log('Custom Master URL: ' + data.d.CustomMasterUrl)
            alert('Custom Master URL: ' + data.d.CustomMasterUrl);

        }
    });
</script>

Add-in Code Snippet

The following code snippet is used in SharePoint Add-in to get the custom master page url of a SharePoint site. This returns a site relative url of a custom master page.

// Load the js files and continue to the successHandler
$.getScript(scriptbase + "SP.RequestExecutor.js", execCrossDomainRequest);

// Function to prepare and issue the request to get
// SharePoint data
function execCrossDomainRequest()
{
    // Initialize the RequestExecutor with the add-in web URL.
    var executor = new SP.RequestExecutor(appweburl);
    // Issue the call against the add-in web.
    // To get the CustomMasterUrl property using REST we can hit the endpoint:
    // appweburl/_api/web?select=CustomMasterUrl&@target=hostweburl
    // The response formats the data in the JSON format.
    executor.executeAsync(
    {
        url: appweburl + "/_api/SP.AppContextSite(@target)/web?$select=custommasterurl&@target='" + hostweburl + "'",
        method: "GET",
        headers:
        {
            "Accept": "application/json; odata=verbose"
        },
        success: successHandler,
        error: errorHandler
    });
}
// Function to handle the success event.
function successHandler(data)
{
    var jsonObject = JSON.parse(data.body)
    //jsonObject.d.CustomMasterUrl returns the server relative url of custom master page from the SharePoint site
    console.log('Custom master page URL: ' + jsonObject.d.CustomMasterUrl)
    alert('Custom master page URL: ' + jsonObject.d.CustomMasterUrl);
}

function errorHandler(data, errorCode, errorMessage)
{
    console.log("Could not complete cross-domain call: " + errorMessage);
}