onsenui/esm/vendor/viewport.js

(function () {
  var DEFAULT_VIEWPORT = 'width=device-width,initial-scale=1,maximum-scale=1,minimum-scale=1,user-scalable=no';

  var Viewport = {
    ensureViewportElement: function ensureViewportElement() {
      var viewportElement = document.querySelector('meta[name=viewport]');

      if (!viewportElement) {
        viewportElement = document.createElement('meta');
        viewportElement.name = 'viewport';
        document.head.appendChild(viewportElement);
      }

      return viewportElement;
    },

    setup: function setup() {
      var viewportElement = Viewport.ensureViewportElement();

      if (!viewportElement) {
        return;
      }

      if (!viewportElement.hasAttribute('content')) {
        viewportElement.setAttribute('content', DEFAULT_VIEWPORT);
      }
    }
  };

  window.Viewport = Viewport;
})();