Question

I am a newbie to java script and currently reading John Resig’s Pro javascript techniques . While explaining closure he refers to calls like setTimeout("otherFunction()",2000) as instances where new JS developers have problems . I could not understand why this is a problem ? Can some one explain please ?In this http://www.w3schools.com/js/js_timing.asp I am seeing a call like var t=setTimeout("alertMsg()",3000); which looks similar to me .

Answer 1

It’s not “wrong”, it’s just not necessarily “right”, and it’s certainly not recommended.

The setTimeout() function‘s first parameter can be a string or a function reference / function expression.

If you pass a string it will be slower because effectively you are doing an eval() which is not recommended. More important than speed though is that the scope in which the code in the string executes may not be what you are expecting (and may not be the same in different browsers).

By passing a function reference / function expression instead these problems can be avoided.

The “right” syntax for your example is:

setTimeout(otherFunction, 2000);

Note there are no parentheses after otherFunction – if there were it would call otherFunction() immediately and pass the return value from that function to setTimeout().

If you need to pass parameters to your function you can wrap it in an anonymous function:

setTimeout(function() {
    otherFunction(param1, param2);
}, 2000);

That may seem kind of clunky compared to setTimeout("otherFunction(param1,param2)", 2000) but again it avoids issues with scope of where otherFunction, param1 and param2 are defined.