Calling Workflow using JavaScript

var clientUrl = Xrm.Page.context.getClientUrl();

var workflowId = "workflow guid";

var entityId = "entityID";

var requestUri = clientUrl + "/api/data/v9.0/workflows(" + workflowId + ")/Microsoft.Dynamics.CRM.ExecuteWorkflow";

var xhr = new XMLHttpRequest();

xhr.open("POST", requestUri, true);

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

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

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

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

xhr.onreadystatechange = function () {

    if (this.readyState == 4) {

        xhr.onreadystatechange = null;

        if (this.status == 200) {

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

        } else {

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

        }

    }

};

xhr.send("{\"EntityId\":\"" + entityId + "\"}");

to get Workflow ID using Name

<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false">

  <entity name="workflow">

    <filter type="and">

      <condition attribute="name" operator="eq" value="Enter Workflow Name Here!" />

    </filter>

  </entity>

</fetch>