Why does adding an array to number return a string?

Question

var array = [1,2,4];

array+1  //gives '1,2,41'.

Can anyone explain this behaviour?

score 4 · Answer 1 · answered Nov 15 '17 at 11:38

4

Array is casted to string - then concatenated with integer value which is also casted to string.

answered Nov 15 '17 at 11:38

hsz

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1

Sorry,but that was neither explanatory not informative. – Lava kumar N R Nov 15 '17 at 15:15
How? It describes what happens, that an explanation. And the informative part is now you know it. A bit short and simple answer but it's on point. – DanteTheSmith Nov 15 '17 at 15:18

score 4 · Accepted Answer · edited Nov 15 '17 at 12:10

4

Can anyone explain this behaviour?

This answer attempts to explain this behavior from the point of view of spec.

As per spec, during the run-time evaluation of +, both expressions (left and right) are converted to their primitive values.

Let lprim be ToPrimitive(lval).

Let rprim be ToPrimitive(rval).

toPrimitive tries to pass hint:number (since invoked during arithmetic evaluation) to OrdinaryToPrimitive

If hint is "string", then
a. Let methodNames be «"toString", "valueOf"».

Else,
b. Let methodNames be «"valueOf", "toString"». //this gets invoked

Since one of the values were casted to string via 4a) above, string concatenation takes place.

Hence

[1,2,4] + 1 => [1,2,4].toString() + "1" => "1,2,4" + "1" => (finally) "1,2,41"

edited Nov 15 '17 at 12:10

Koby Douek

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answered Nov 15 '17 at 11:50

gurvinder372

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I loved how you pulled exact place when that is happening an why. I know why it does so but I never thought about looking where is that written in such a manner. +1 – DanteTheSmith Nov 15 '17 at 11:54
@DanteTheSmith Thanks – gurvinder372 Nov 15 '17 at 11:56
@gurvinder372 Edited your question to match the OP's example, and +1 from me :) – Koby Douek Nov 15 '17 at 12:11
@KobyDouek Thanks, didn't notice that earlier. – gurvinder372 Nov 15 '17 at 12:12
Thanks @gurvinder372 .. much helpful :) – Lava kumar N R Nov 15 '17 at 15:17

Koby Douek · Answer 3 · 2017-11-15T12:15:49.627

2

When you use the + sign with a declared javascipt object (var array), even if one of the elements is a number, it doesn't perform an arithmetic addition operation - it concatenates the values as two strings.

In your example, your array [1,2,4] is being casted into a string with a value of 1,2,4. So 1,2,4 concatenated with 1 is 1,2,41

edited Nov 15 '17 at 12:15

answered Nov 15 '17 at 11:39

Koby Douek

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1

Except when you don't declare that object first `{} + 1` equals `1` but `var obj = {}; obj + 1` will equal `"[object Object]1"` and `{} + {} + 1` equals `NaN` – George Nov 15 '17 at 11:41

score 0 · Answer 4 · answered Nov 15 '17 at 11:44

What did you expect? [2,3,5]?

You haven't wrote a mutator for array, you added 1 to array (which is an object). Why do you expect for object to be able to add 1 to it?

JS figured out you need a primitive from that object and listed that object into a string. Now it knows how to "add" 2 strings (precisely its concatenate) so it did.

If you expected an entire array to get +1 on all elements. You want:

for (var i=array .length; i--;) {
    array [i]++;
}

Or

array = array.map(function(e) {return '#' + e});

Or in ES6 and beyond arrow function with map

array = array.map(i => i + 1);

Question was purely regarding the behaviour not on pushing the value in the array nor adding it to each element. Thanks for the effort anyway — Lava kumar N R, Nov 15 '17 at 15:16

score 0 · Answer 5 · answered Nov 15 '17 at 11:47

When the operands of an operator are different types, they will be converted to a common type.

Both an array and a Number can be converted to string, this is called coercion.

If you would like to add 1 to an array, you can do this:

var array = [1,2,4];
array.push(1);

Why does adding an array to number return a string?

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