Useful JS Tips

#75 - Upping Performance by Appending/Keying

2017-05-29 by @loverajoel

React uses a mechanism called reconciliation for efficient rendering on update.

Reconciliation works by recursively comparing the tree of DOM elements, the specifics of the algorithm might be found on the official documents or the source, which is always the best source.

This mechanism is partly hidden, and as such, there are some conventions that might be followed to ease its inner workings.

One such example is appending vs inserting.

Reconciliation compares the list of root’s nodes children at the same time, if two children differ, then React will mutate every one.

So, for example, if you’ve got the following:

<ul>
    <li>Sherlock Holmes</li>
    <li>John Hamish Watson</li>
</ul>

And you insert an element at the beginning

<ul>
    <li>Mycroft Holmes</li>
    <li>Sherlock Holmes</li>
    <li>John Hamish Watson</li>
</ul>

React will start of by comparing the old tree, where <li>Second</li> is the first child and the new tree where <li>First</li> is the new first child. Because they are different, it will mutate the children.

If, instead of inserting good Mycroft you appended him, React would perform better, not touching Sherlock nor John.

But you can’t always append, sometimes you’ve just got to insert.

This is were keys come in. If you supply a key attribute then React will figure out an efficient transformation from the old tree to the new one.

<ul>
    <li key="3">Mycroft Holmes</li>
    <li key="1">Sherlock Holmes</li>
    <li key="2">John Hamish Watson</li>
</ul>
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