javascript - why undefined in the function of addEventListener? -

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this question has answer here:

i'm trying add mouseenter event listener elements change childnodes' opacity. use for add listener, , can output childnodes properly, in function it's "undefined". have no idea why.

html:

<article class="index-image-article rel">     <section class="index-image-info">         // elements      </section>      <div class="index-image-excerpt mask table">          // elements      </div> </article>

javascript:

var indeximagearticle = document.queryselectorall(".index-image-article"); var indeximageinfo = document.queryselectorall(".index-image-info"); var indeximageexcerpt = document.queryselectorall(".index-image-excerpt");  for(var = 0; < indeximagearticle.length; i++) {     console.log(indeximageinfo[i]); // output     indeximagearticle[i].addeventlistener("mouseenter", function() {         indeximageinfo[i].style.opacity = 1;         indeximageexcerpt[i].style.opacity = 0;     }); }

the output of console.log is:

<section class=​"index-image-info">​…​</section>​ <section class=​"index-image-info">​…​</section>​ <section class=​"index-image-info">​…​</section>​ <section class=​"index-image-info">​…​</section>

and error:

uncaught typeerror: cannot read property 'style' of undefined(anonymous function)

​the undefined point indeximageinfo[i]

this 1 of strange nuances of variable scoping rules in javascript.

really boils down 1 thing, @ point of adding event handler, i has right value. @ point of executing event handler i has been incremented indeximagearticle.length, , therefore outside of bounds of array.

it can fixed using block maintains scope of variable, iife 1 way:

for(var = 0; < indeximagearticle.length; i++) {     console.log(indeximageinfo[i]); // output     (function(x){         indeximagearticle[x].addeventlistener("mouseenter", function() {             indeximageinfo[x].style.opacity = 1;             indeximageexcerpt[x].style.opacity = 0;         });     })(i) }

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