Window: length property

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.

Returns the number of frames (either <frame> or <iframe> elements) in the window.

A number.

if (window.length) {
  // this is a document with subframes
}

Specification
HTML Standard # dom-length-dev

BCD tables only load in the browser