Window.length

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

window의 frame 개수를 반환합니다. (<frame> 또는 <iframe> 요소들 중 하나)

신텍스

framesCount = window.length;

if (window.length) {
  // document의 subframes
}

Specification
HTML # dom-length-dev