CRM 2016 - WEB API Operations - Associate/Disassociate for Many to Many

Associate and Disassociate for Many to Many (i.e. Collection-Valued navigation property) relationships using the new CRM 2016 Web API in Javascript.
In this example I am going to show you how to Associate/Disassociate two out of the box records: an opportunity and an account.
You can find more information about Single and Collection-Valued properties here and here

this.associateRecords = function (parentId, parentType, relationshipName, childId, childType, successCallback, errorCallback) {
        var req = new XMLHttpRequest();
        req.open("POST", encodeURI(getWebAPIPath() + parentType + "(" + parentId + ")/" + relationshipName + "/$ref"), true);
        req.setRequestHeader("Accept", "application/json");
        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
        req.setRequestHeader("OData-MaxVersion", "4.0");
        req.setRequestHeader("OData-Version", "4.0");
        req.onreadystatechange = function () {
            if (this.readyState == 4 /* complete */) {
                req.onreadystatechange = null;
                if (this.status == 204 || this.status == 1223) {
                    successCallback();
                }
                else {
                    errorCallback(bd_Utilities.WebAPI.errorHandler(this));
                }
            }
        };
        var childEntityReference = { "@odata.id": getWebAPIPath() + "/" + childType + "(" + childId + ")" };

        req.send(JSON.stringify(childEntityReference));  
    };

//Account: E48456EE-49B0-E611-810F-3863BB357C39
//Opportunity: F45D2DF7-6DBF-E511-8109-3863CB343C91
associate: function () {
        bd_Utilities.WebAPI.associateRecords("E48333EE-49B9-E511-810F-3863BB357C38", "accounts", "opportunity_customer_accounts", "F25D2DF7-6CBF-E511-8109-3863BB343C90", "opportunities",
            function () {

            },
            function () {

            });
    }

And the Disassociate

this.disassociateRecords = function (parentId, parentType, relationshipName, childId, childType, successCallback, errorCallback) {
        var req = new XMLHttpRequest();
     
        var webApiPath = getWebAPIPath();
        var parentUri = webApiPath + "/" + parentType + "(" + parentId + ")/";
        var childUri = webApiPath + "/" + childType + "(" + childId + ")";

        req.open("DELETE", encodeURI(parentUri + relationshipName + "/$ref?$id=" + childUri), true);

        req.setRequestHeader("Accept", "application/json");
        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
        req.setRequestHeader("OData-MaxVersion", "4.0");
        req.setRequestHeader("OData-Version", "4.0");
        req.onreadystatechange = function () {
            if (this.readyState == 4 /* complete */) {
                req.onreadystatechange = null;
                if (this.status == 204 || this.status == 1223) {
                    successCallback();
                }
                else {
                    errorCallback(bd_Utilities.WebAPI.errorHandler(this));
                }
            }
        };
        req.send(JSON.stringify());
    };

function getWebAPIPath() {
        return getClientUrl() + "/api/data/v8.0/";
}

function getClientUrl() {
        //Get the organization URL
        if (typeof GetGlobalContext == "function" &&
            typeof GetGlobalContext().getClientUrl == "function") {
            return GetGlobalContext().getClientUrl();
        }
        else {
            //If GetGlobalContext is not defined check for Xrm.Page.context;
            if (typeof Xrm != "undefined" &&
                typeof Xrm.Page != "undefined" &&
                typeof Xrm.Page.context != "undefined" &&
                typeof Xrm.Page.context.getClientUrl == "function") {
                try {
                    return Xrm.Page.context.getClientUrl();
                } catch (e) {
                    throw new Error("Xrm.Page.context.getClientUrl is not available.");
                }
            }
            else { throw new Error("Context is not available."); }
        }
    }