Observe mutations on a target node that doesn’t exist yet

Only an existing node can be observed.

But don’t worry, since getElementById is insanely fast compared to enumeration of all mutations’ added nodes, waiting for the element to appear won’t be taxing at all as you will see in Devtools -> Profiler panel.

function waitForAddedNode(params) {
    new MutationObserver(function(mutations) {
        var el = document.getElementById(params.id);
        if (el) {
            this.disconnect();
            params.done(el);
        }
    }).observe(params.parent || document, {
        subtree: !!params.recursive || !params.parent,
        childList: true,
    });
}

Usage:

waitForAddedNode({
    id: 'message',
    parent: document.querySelector('.container'),
    recursive: false,
    done: function(el) {
        console.log(el);
    }
});

Always use the devtools profiler and try to make your observer callback consume less than 1% of CPU time.

• Whenever possible observe direct parents of a future node (subtree: false)
• Use getElementById, getElementsByTagName and getElementsByClassName inside MutationObserver callback, avoid querySelector and especially the extremely slow querySelectorAll.
• If querySelectorAll is absolutely unavoidable inside MutationObserver callback, first perform the querySelector check, on the average such combo will be much faster.
• Don’t use Array methods like forEach, filter, etc. that require callbacks inside MutationObserver callback because in Javascript function invocation is an expensive operation compared to the classic for (var i=0 ....) loop, and MutationObserver callback may fire 100 times per second with dozens, hundreds or thousands of addedNodes in each batch of mutations on complex modern pages.
• Don’t use the slow ES2015 loops like for (v of something) inside MutationObserver callback unless you transcompile and the resultant code runs as fast as the classic for loop.