That is not a repo of the code in the article. The article's code is effectively

    set_href('/foo');

    let future = doSomethingElse()
    block_on(future)

    if (some_condition) {
        set_href('/bar');
    }

This code makes the bug clearer. doSomethingElse is effectively allowing the page to exit. this would be no different in many apps, even in rust.

The browser does not start a process when you set `window.location.href`. It starts a process after your code exits and lets the event loop run other tasks. The `await` in the example code is what allow other tasks to run, including the task to load a new page, (or quit an app, etc..) That task that was added when you set `window.location.href`

If that's not clear

    // task 1
    window.location.href = '/foo' // task2 (queues task2 to load the page)

    let content = await response.json(); // adds task3 to load json
                                         // which will add task4
                                         // to continue when finished

    // task4
    if (content.onboardingDone) {
        window.location.href = "/dashboard";
    } else {
        window.location.href = "/onboarding";
    }

task2 runs after task1. task1 exits at the `await`. task2, clears out all the tasks. task3 and task4 never run.