iOS how to run iphone function after javascript finished?

Hi guys. I call some step below:

//Step1: call a javascript function.
NSString *returnResult = [webView stringByEvaluatingJavaScriptFromString:_scriptMethod];

//Step2: alert return result:
[self alerMessage: returnResult];

The step2 is always called while step1 still does not finish yet. So alert message is empty. (Because of iphone function speed faster than javascript function).

Please help me to run step3 after step1 finish.

Thank you so much.