CODEWITHSUNDEEP
javascriptjquery
Ikhlak S.
Ikhlak S.

Reputation: 9044

Calling `Function` vs `Function()` within the same function

I was writing a long polling script and ran into a too much recursion error which hung the browser. My goal is to call the same function every 1000ms using setTimeout(). Yes, I could use setInterval() but it is going to be a long polling script and will be waiting for a server response.

I fixed this by removing the () from the function I was calling within the same function.

My script looks like:

function messagePolling(){
    console.log("polled")
    setTimeout(messagePolling(),1000) // <--- removing `()` from the function works as intended 
} 

messagePolling();

What is the logic behind this? messagePolling is a function after all isn't it.

Upvotes: 0

Views: 89

Answers (4)

Taiti
Taiti

Reputation: 377

Anywhere a function name contains "()" it is executed immediately except when it is wrapped in quotes i.e is a string.

Upvotes: 0

XCS
XCS

Reputation: 28177

Written as setTimeout(messagePolling(),1000) the function is executed immediately and a setTimeout is set to call undefined (the value returned by your function) after one second. (this should actually throw an error if ran inside Node.js, as undefined is not a valid function)

Written as setTimeout(messagePolling,1000) the setTimeout is set to call your function after one second.

Upvotes: 1

jlogan
jlogan

Reputation: 947

When you type messagePolling you are passing the function to setTimeout as a parameter. This is the standard way to use setTimeout.

When you type messagePolling() you are executing the function and passing the return value to setTimeout

That being said, this code looks odd to me. This function just runs itself. It's going to keep running itself indefinitely if you do this.

Upvotes: 0

Nebula
Nebula

Reputation: 7151

You're absolutely right - messagePolling is a function. However, messagePolling() is not a function. You can see that right in your console:

// assume messagePolling is a function that doesn't return anything
messagePolling() // -> undefined

So, when you do this:

setTimeout(messagePolling(), 1000)

You're really doing this:

setTimeout(undefined, 1000)

But when you do this:

setTimeout(messagePolling, 1000)

You're actually passing the function to setTimeout. Then setTimeout will know to run the function you passed - messagePolling - later on. It won't work if it decides to call undefined (the result of messagePolling()) later, right?

Upvotes: 5

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