UIWebView stringByEvaluatingJavaScriptFromString遇到了麻烦

我在视图控制器中有以下代码. “pageDisplay”是一个UIWebView.
当我在模拟器中运行应用程序时,HTML页面出现在JS运行之前. id为“myHeader”的元素,< h1>标签,没有变化.

-(void) loadPageToView:(int)pagenumber{
    NSString *path = [[NSBundle mainBundle] pathForresource:[NSString StringWithFormat:@"%d",pagenumber]ofType:@"html"];
    NSURL *url = [NSURL fileURLWithPath:path];
    NSURLrequest *request = [NSURLrequest requestWithURL:url];
    [pageDisplay loadrequest:request];
    [self doJavaScript];
}

-(void) doJavaScript{
    [pageDisplay    StringByEvaluaTingJavaScriptFromString:@"document.getElementById('myHeader').innerHTML = \"FOO\";"];   
}