Check for dependencies concurrently while the page loads.

dependency-check.js

// Set the window object to be set at a later time then the promise resolve.
setTimeout(() => {
  window.DEP = 'I am here!';
}, 31000)

async function dependencyReady() {
  // Set a timeout limit of 30 seconds.
  const time = Date.now() + 30000;
  // Runs an until time is past and window.DEP is ready.
  while (!Object.prototype.hasOwnProperty.call(window, 'DEP')) {
    try {
      await new Promise((resolve, reject) => {
        setTimeout(resolve, 100);
        
        if (time < Date.now()) {
          // Reject the promise if our timeout limit of 30 seconds is reached.
        	reject('Timeout error. window.DEP is undefined');
        }
      });
    } catch (e) {
      // Break the loop and display a console error.
      console.error(e);
      break;
    }
  }
};

// Check the existence of the dependency.
dependencyReady().then(() => {
  console.log(window.DEP);
});