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Editorial Team
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Asked: 2026-06-11T17:19:31+00:00

HTML : <body> <canvas id=canvas1></canvas> <canvas id=canvas2></canvas> </body> Javascript : window.onload = function() {

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HTML :

<body>
  <canvas id="canvas1"></canvas>
  <canvas id="canvas2"></canvas>
</body>

Javascript :

window.onload = function() {
  var canvasElements = document.getElementsByTagName("canvas");
  for ( var index in canvasElements) {
  }
  for ( var index = 0; index < canvasElements.length; index++) {
  }
}

These two loop does not have same iteration number. Do you know why ?

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1 Answer

  1. Editorial Team

    document.getElementsByTagName("canvas") returns an HTMLCollection that have one propertie, “length” and two methods, “item” and “namedItem”.

    So the FOR … IN loop iterate over items in that collection and over the members of the HTMLCollection.
    So There is 5 iterations:

    • 2 iterations for each HTMLElement, here canvasElement
    • 2 iterations for eahc methods : item, namedItem
    • 1 iteration for the propertie : length

    The for(…;…;…) loop only iterate over items because document.getElementsByTagName(“canvas”)[index] refere to an item

    Hope this response was clear !
    Ask for more information otherwise…

    Some references :

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