Gets the ID of the underlying operation.

import Batch = require("nativescript-batch");

var b = Batch.newBatch(function(ctx) {
    console.log("ID: " + ctx.id);  // operation-1
});
b.id = "operation-1";

b = b.then(function(ctx) {
    console.log("ID: " + ctx.id);  // operation-2
});
b.id = "operation-2";

b.start();