Measures the duration of a 2-second simulated operation after page `load` using `performance.now()`

measure-operation-duration.js

// Function to simulate
// a long-running operation
function longRunningOperation() {
  // Simulate a 2-second operation
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log(
        "Operation completed"
      );
      resolve();
    }, 2000);
  });
}

// Measure the time taken
// for the long-running
// operation after the `load`
window.addEventListener(
  "load",
  async () => {
    console.log("Page loaded");

    // Start measuring time
    const startTime = performance.now();

    // Perform the long-running operation
    await longRunningOperation();

    // Stop measuring time
    const endTime = performance.now();

    // Calculate the duration
    // of the operation
    const duration =
      endTime - startTime;
    console.log(
      `Operation took ${duration} milliseconds`
    );
  }
);