To get the size of an array in an object with JavaScript, we use the length property.

For instance, we write

const st = {
  itemA: {},
  itemB: [
    { id: "s01", cd: "c01", dd: "d01" },
    { id: "s02", cd: "c02", dd: "d02" },
  ],
};

console.log(st.itemB.length);

to get the length of the st.itemB array property with st.itemB.length.