Put a Delay in Javascript

If you're okay with ES2017, await is good:

const DEF_DELAY = 1000;

function sleep(ms) {
  return new Promise(resolve => setTimeout(resolve, ms || DEF_DELAY));
}

await sleep(100);

Note that the await part needs to be in an async function:

//IIAFE (immediately invoked async function expression)
(async()=>{
  //Do some stuff
  await sleep(100);
  //Do some more stuff
})()

Unfortunately, setTimeout() is the only reliable way (not the only way, but the only reliable way) to pause the execution of the script without blocking the UI.

It's not that hard to use actually, instead of writing this:

var x = 1;

// Place mysterious code that blocks the thread for 100 ms.

x = x * 3 + 2;
var y = x / 2;

you use setTimeout() to rewrite it this way:

var x = 1;
var y = null; // To keep under proper scope

setTimeout(function() {
    x = x * 3 + 2;
    y = x / 2;
}, 100);

I understand that using setTimeout() involves more thought than a desirable sleep() function, but unfortunately the later doesn't exist. Many workarounds are there to try to implement such functions. Some using busy loops:

function sleep(milliseconds) {
  var start = new Date().getTime();
  for (var i = 0; i < 1e7; i++) {
    if ((new Date().getTime() - start) > milliseconds){
      break;
    }
  }
}

others using an XMLHttpRequest tied with a server script that sleeps for a amount of time before returning a result.

Unfortunately, those are workarounds and are likely to cause other problems (such as freezing browsers). It is recommended to simply stick with the recommended way, which is setTimeout()).

I just had an issue where I needed to solve this properly.

Via Ajax, a script gets X (0-10) messages. What I wanted to do: Add one message to the DOM every 10 Seconds.

the code I ended up with:

$.each(messages, function(idx, el){
  window.setTimeout(function(){
    doSomething(el);
  },Math.floor(idx+1)*10000);
});

Basically, think of the timeouts as a "timeline" of your script.

This is what we WANT to code:

DoSomething();
WaitAndDoNothing(5000);
DoSomethingOther();
WaitAndDoNothing(5000);
DoEvenMore();

This is HOW WE NEED TO TELL IT TO THE JAVASCRIPT:

At Runtime 0    : DoSomething();
At Runtime 5000 : DoSomethingOther();
At Runtime 10000: DoEvenMore();

Hope this helps.