Defer a JavaScript function

Oftentimes, non-critical tasks can be deferred to improve the responsiveness of a web application. This can be achieved by deferring the invocation of a function until the current call stack has been cleared. This is particularly useful when you want to update the UI before running a long-running function.

Using setTimeout() with a timeout of 1 ms, we can defer the invocation of a function until the current call stack has been cleared. As a result, the browser will update the UI before running the function. We can also use the spread (...) operator to supply the function with an arbitrary number of arguments.

const defer = (fn, ...args) => setTimeout(fn, 1, ...args);

// Example A:
defer(console.log, 'a'), console.log('b'); // logs 'b' then 'a'

// Example B:
document.querySelector('#someElement').innerHTML = 'Hello';
longRunningFunction();
// Browser will not update the HTML until this has finished
defer(longRunningFunction);
// Browser will update the HTML then run the function