Window.frames

Возвращает само окно, являющееся объектом в виде массива, распечатывая элементы текущего окна.

frameList = window.frames;

• frameList is a list of frame objects. It is similar to an array in that it has a length property and its items can be accessed using the [i] notation.
• frameList === window evaluates to true.
• Each item in the window.frames pseudo-array represents the window object corresponding to the given <frame>'s or <iframe>'s content, not the (i)frame DOM element (i.e., window.frames[0] is the same thing as document.getElementsByTagName("iframe")[0].contentWindow).
• For more details about the returned value, refer to this thread on mozilla.dev.platform [dead link].

var frames = window.frames; // or // var frames = window.parent.frames;
for (var i = 0; i < frames.length; i++) {
  // do something with each subframe as frames[i]
  frames[i].document.body.style.background = "red";
}