Dynamics 365 CRM retrieve data using JavaScript

What is OData in Dynamics CRM?

Open Data Protocol (OData) is a standard protocol for consuming data exposed by Dynamics. OData is a Representational State Transfer (REST) based protocol. It can be applied to all types of technologies, including JavaScript Object Notation (JSON).

To retrieve single record data using JavaScript

function CheckPrimaryContactId(executionContext) {

    formContext = executionContext.getFormContext();

    var primarycontactid = formContext.data.entity.attributes.get("primarycontactid");

    if (primarycontactid.getValue() != null) {

        entityId = primarycontactid.getValue()[0].id.slice(1, -1);

        entityName = primarycontactid.getValue()[0].entityType;

        entityLabel = primarycontactid.getValue()[0].name;

        var globalContext = Xrm.Utility.getGlobalContext();

        var crmURL=globalContext.getClientUrl()

        var req = new XMLHttpRequest();

        req.open("GET",crmURL+"/api/data/v9.1/contacts("+entityId+")?$select=emailaddress1",false);

        req.setRequestHeader("OData-MaxVersion", "4.0");

        req.setRequestHeader("OData-Version", "4.0");

        req.setRequestHeader("Accept", "application/json");

        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");

        req.setRequestHeader("Prefer", "odata.include-annotations=\"*\"");

        req.onreadystatechange = function () {

            if (this.readyState === 4) {

                req.onreadystatechange = null;

                if (this.status === 200) {

                    var resultsData = JSON.parse(this.response);

                    if (resultsData) {

                        emailaddress1 = resultsData["emailaddress1"];

                        console.log("Email Address is:" + emailaddress1);

                    }

                } else { Xrm.Utility.alertDialog(this.statusText); }

            }

        };

        req.send();

    }

}