Question about JS method call brackets

The question of using the JS method to call brackets is a headache. The following is a good example. If you are interested, refer

On the mobile html page, a function is written. the Code is as follows: function showAlert (msg, fn) {showDialog ("alert", msg, "tip", 260); if (isNull (fn) = false) {$ ("# SD_confirm "). unbind ("click"); $ ("# SD_confirm "). bind ("click", fn) ;}} function exist () {alert ("aaa");} fn is a function, during the call, I wrote the copy code as follows: showAlert (json. msg, exist (); when the result reaches this line of code, before the showDialog method is reached, exit exist to bring up aaa. What happened, and then I found that I was wrong. the Code should be written as follows: showAlert (json. msg, exist); then, a prompt for dialog appears, and click OK. The error "aaa. Mei" is displayed. It turns out to be an error in parentheses. Khan!