Optimisation #1 Javascript : Mise en cache de la longueur d’un array

Pour optimiser un peu plus son code Javascript (pour ceux qui n’appliquent pas encore cette astuce bien sûr), je vais partager une petite best practice lors de l’itération d’une boucle for.

La boucle for « normale » ressemble à ça…

var tab = [‘element1’, ‘element2’, ‘element3’];
for (var i= 0 ; i<tab.length ; i++) {
// le reste du code

Dans le code ci-dessus on constate qu’à chaque itération, la taille du tableau (tab) est recalculée via la commande tab.length. Mauvais!
Le but est de calculer cette longueur qu’une seule fois :

var tab = [‘element1’, ‘element2’, ‘element3’];
for (var i= 0, iLen = tab.length ; i<iLen ; i++) {
// le reste du code

Dans le code ci-dessus, on constante donc que la longueur est calculée la première fois et qu’elle est ensuite stockée dans la variable iLen.

Pour des tableaux avec seulement quelques éléments l’optimisation ne se fait pas ressentir mais quand on travaille sur des milliers de lignes c’est plutôt pas mal!

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