console.log outputs 'function ' instead of ''

Question

I am reading the javascript manual and I have wtote following code:

//sum
function sum(arg1) {
    var sum = arg1;

    function f(arg2) {
        sum += arg2;
        return f;
    };

    f.valueOf = function () {
        return sum;
    };

    f.toString = function () {
        return sum;
    };

    return f;
}

and I execute it like this:

console.log(sum(1)(2)(3)(4));

According the manual console.log should output the result of valueOf function

but it output

function 10

please explain this behaviour.

do you mean the word function in the output? it could be the beaviour of the console. — Nina Scholz, Nov 24 '16 at 08:40

T.J. Crowder · Accepted Answer · 2016-11-24T08:46:16.493

3

According the manual console.log should output the result of valueOf function

I don't know what "manual" you're talking about, but in general, console.log doesn't call valueOf on what you output. The implementations of console.log vary from JavaScript engine to JavaScript engine, but in general they try to give you more useful information than calling valueOf would.

If you want to trigger valueOf, you'll need to do that intentionally, for instance:

console.log(sum(1)(2)(3)(4).valueOf());
// ------------------------^^^^^^^^^^

or (since your function is designed to produce a number):

console.log(+sum(1)(2)(3)(4));
// ---------^

Example:

function sum(arg1) {
    var sum = arg1;

    function f(arg2) {
        sum += arg2;
        return f;
    };

    f.valueOf = function () {
        return sum;
    };

    f.toString = function () {
        return sum;
    };

    return f;
}

console.log(sum(1)(2)(3)(4).valueOf());
// ------------------------^^^^^^^^^^

console.log(+sum(1)(2)(3)(4));
// ---------^

Look in the actual browser console; the Stack Snippets console doesn't quite do what the real browser console does.

edited Nov 24 '16 at 08:46

answered Nov 24 '16 at 08:40

T.J. Crowder

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with + all works properly, but I am not sure which object can I return to get this behaviour – gstackoverflow Nov 24 '16 at 08:46
where did you get the idea of adding a + sign on the start of it? can you link it or explain. thanks :D – Semi-Friends Nov 24 '16 at 08:50
2

@Semi-Friends + means cast to number – gstackoverflow Nov 24 '16 at 08:55
1

@Semi-Friends http://stackoverflow.com/questions/1133770/how-do-i-convert-a-string-into-an-integer-in-javascript – gstackoverflow Nov 24 '16 at 08:56
@gstackoverflow: I'm sorry, I don't know what you mean by *"but I am not sure which object can I return to get this behaviour"*. – T.J. Crowder Nov 24 '16 at 09:01
@T.J. Crowder, please provide code which approximately equals code in engine – gstackoverflow Nov 24 '16 at 09:35
@gstackoverflow: Again, I have no idea what you mean by that. The above does what you said you want it to do. You need to do *something* with the returned function that invokes `valueOf`, otherwise it's a reference to the function (which your code relies on when calling it with `(2)`). – T.J. Crowder Nov 24 '16 at 10:04

console.log outputs 'function ' instead of ''

1 Answers1