UserScript utilities

user-script.util.js

/** returns a promise that resolves when element becomes available */
function waitForElement(selector, timeout=1000) {
  return new Promise((resolve, reject) => {
    let start, element;
    requestAnimationFrame(function wait(timestamp) {
      if (start == null) start = timestamp;
      element = document.querySelector(selector);
      if (element) {
        resolve(element);
      } else {
        const elapsed = timestamp - start;
        if (elapsed < timeout) {
          requestAnimationFrame(wait)
        } else {
          reject(`timed out after ${elapsed}ms`)
        }
      }
    })
  })
}