Coding style: utilize default values

Since they are a language feature, they make the source code more easily
comprehensible than `if (typeof XXX === 'undefined') { XXX = ...; }`.

react/features/base/dialog/components/AbstractDialog.js

      * @returns {void}
      */
     _onCancel() {
-        const { cancelDisabled, onCancel } = this.props;
+        const { cancelDisabled = false, onCancel } = this.props;
 
-        if ((typeof cancelDisabled === 'undefined' || !cancelDisabled)
-                && (!onCancel || onCancel())) {
+        if (!cancelDisabled && (!onCancel || onCancel())) {
             this._hide();
         }
     }
      * @returns {void}
      */
     _onSubmit(value: ?string) {
-        const { okDisabled, onSubmit } = this.props;
+        const { okDisabled = false, onSubmit } = this.props;
 
-        if (typeof okDisabled === 'undefined' || !okDisabled) {
+        if (!okDisabled) {
             this.setState({ submitting: true });
 
             // Invoke the React Compnent prop onSubmit if any.

react/features/base/react/components/web/Container.js

      * @returns {ReactElement}
      */
     render() {
-        const { visible } = this.props;
+        const { visible = true } = this.props;
 
-        return (
-            typeof visible === 'undefined' || visible
-                ? super._render('div')
-                : null);
+        return visible ? super._render('div') : null;
     }
 }

react/features/calendar-sync/functions.native.js

  * otherwise, {@code false}.
  */
 export function isCalendarEnabled() {
-    const { calendarEnabled } = NativeModules.AppInfo;
+    const { calendarEnabled = true } = NativeModules.AppInfo;
 
-    return typeof calendarEnabled === 'undefined' ? true : calendarEnabled;
+    return calendarEnabled;
 }
 
 /**

react/features/subtitles/components/AbstractClosedCaptionButton.js

 export function _abstractMapStateToProps(state: Object, ownProps: Object) {
     const { _requestingSubtitles } = state['features/subtitles'];
     const { transcribingEnabled } = state['features/base/config'];
-
-    // Default 'visible' to 'transcribingEnabled' if not explicitly specified
-    // in the component's properties.
     const { visible = transcribingEnabled } = ownProps;
 
     return {