Function calling in Javascript with double brackets

Question

When I call function hi()() with double brackets the function displays hi output and it will also give error saying, that hi is not function.

<html>
    <head></head>
<script>

function hello()
{
    document.write("hello");
}
function hi()
{
    document.write("hi");
    return "hello";
}
hi()();
</script>
</html>

What is the meaning of using ()() with function name?

Please edit the title to specify your question is about double parenthesis, not double brackets. — volpato, Apr 13 '21 at 13:29

score 13 · Accepted Answer · answered Sep 14 '13 at 13:03

13

The double parenthesis would have been useful if hi had returned a function instead of its name, like in

function hi(){
    return hello;
}
hi()();

That's probably what was the intent.

answered Sep 14 '13 at 13:03

Denys Séguret

372,613
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1

Or `window[hi()]();` :P – Oleg Sep 14 '13 at 13:05
1

@Oleg Yes, but that wouldn't make it better. – Denys Séguret Sep 14 '13 at 13:05

score 11 · Answer 2 · answered Sep 14 '13 at 13:13

Putting () after something that evaluates to a function will call that function. So, hi() calls the function hi. Assuming hi returns a function then hi()() will call that function. Example:

function hi(){
    return function(){return "hello there";};
}

var returnedFunc = hi();  // so returnedFunc equals function(){return "hello there";};
var msg = hi()();         // so msg now has a value of "hello there"

If hi() doesn't return a function, then hi()() will produce an error, similar to having typed something like "not a function"(); or 1232();.

score 10 · Answer 3 · answered Apr 17 '17 at 12:57

10

()() means calling a function and if returns another function second parenthesis will call it.Please find below example :

function add(x){
    return function(y){
        return x+y;
    }
}

add(3)(4)

output: 7

in above case add(4) will be called for add function and add(3) will be called for returned function. here value of parameter x is 3 and parameter y is 4.

Please note : we use parenthesis for function call.

answered Apr 17 '17 at 12:57

RVCoder

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4
14

Any idea we can perform dynamic parameter using rest parameter in the same way. I mean add(3)(4) or add(3)(4)(5) or any number of brackets we add it should perform that add operation. How to achieve this ? – Santosh Feb 10 '21 at 15:21
@Santosh that would be done using recursion – Muhammad Awais Aug 07 '22 at 14:55
BTW. It can be done by using currying method. – Santosh Aug 08 '22 at 02:06

frogatto · Answer 4 · 2015-10-08T12:17:00.223

The return value of this function is a string which is not a callable object.

function hi()
{
    document.write("hi");
    return "hello"; // <-- returned value
}

But if you want to call this function multiple times you can use a for-loop or some things else.

Example of hi()():

function hi(){
    return function(){ // this anonymous function is a closure for hi function
       alert('some things')
    }
}

JS Fiddle: here

If you want to call hello function immediately after hi try this:

 function hi()
    {
        document.write("hi");
        return hello; //<-- no quote needed
        // In this context hello is function object not a string
    }

score 0 · Answer 5 · answered Sep 14 '13 at 13:18

0

You can use eval() to execute it even if it's string : eval(hi()+'()');

answered Sep 14 '13 at 13:18

Charaf JRA

8,249
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a less evil way of doing the same thing would be `window[hi()]()` – Stuart Sep 14 '13 at 13:27

Function calling in Javascript with double brackets

5 Answers5

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