How to call a function that returns another function in JavaScript? [duplicate]

Question

I was taking a small test online and there was this code:

function getFunc() {
    var a = 7;
    return function(b) {
        alert(a+b);
    }
}
var f = getFunc();
f(5);

I would like to know why I can't call getFunct(5) directly for example.

I don't understand the last two lines.

Why do I need to assign the function to a variable. What happens when doing f(5)?

How does JS interpret that the 5 is a variable for the inner function and not the outer?

Because that's not the definition of getFunc. getFunc returns a function you call with a parameter. Look at the signature of getFunc: is there a parameter? — Dave Newton
– Dave Newton, Commented Apr 29, 2019 at 14:42
getFunc returns a function. So, f is just the inner function where the contextual a is 7. You shouln't directly call it, because the call signature of getFunc just doesn't accept any argument, so calling it with any argument will do nothing. — briosheje
– briosheje, Commented Apr 29, 2019 at 14:42
@briosheje Well. You could, it just wouldn't work the way the OP wants it to. — Dave Newton
– Dave Newton, Commented Apr 29, 2019 at 14:43
@DaveNewton You're right, you technically could indeed, but that would do nothing. However, that's right, you actually can do that, but it just won't do anything. — briosheje
– briosheje, Commented Apr 29, 2019 at 14:44

Nina Scholz · Accepted Answer · 2019-04-29 14:43:31Z

7

You could call the inner function immediately after calling the first one, because the first call returns a function and the second call gives the result.

function getFunc() {
    var a = 7;
    return function(b) {
        console.log(a + b);
    }
}

getFunc()(5);

edited Apr 29, 2019 at 14:43

answered Apr 29, 2019 at 14:42

Nina Scholz

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3 Comments

Andreas Over a year ago

"I don't understand the last two lines. Why do I need to assign the function to a variable. What happens when doing f(5)? How does JS interpret that the 5 is a variable for the inner function and not the outer?"

Dave Newton Over a year ago

Yes, but I think the OP is asking why it is this way, not a way to eliminate the intermediate variable.

user3808307 Over a year ago

Yes @DaveNewton this is exactly what I needed. Thank you

Swanand · Accepted Answer · 2019-04-29 14:51:58Z

3

By assigning getFunc() to the variable f you actually assigned the return value i.e. inner function to f since that's what getFunc is returning. The braces () make a difference here.

However had it been f = getFunc i.e. without the braces, that would imply f is an alias for getFunc and you'd have to do f()(5) in that case.

edited Apr 29, 2019 at 14:51

answered Apr 29, 2019 at 14:43

Swanand

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