How to call AlertIOS in React Native

I am an iOS developer, but I know a little javascript. I am trying to use the AlertIOS, the document api is this

static alert(title: string, message?: string, buttons?: Array<{ text: ?string; onPress: ?Function; }>) 

I am confused with the parameters. I tried to write like this,but it give me error.AlertIOS('Username empty', 'Please type your username', buttons: {{text: 'Cancel', onPress: onPressCancel}});

How can i use AlertIOS properly ?