Window: свойство 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.

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Возвращает число фреймов (как <frame> так и <iframe> ) в окне.

Значение

Число.

if (window.length) {
  // этот документ содержит фреймы
}

Specification
HTML # dom-length-dev