Why is number + string a string in javascript?

Question

Sort of trying out some quirks in javascript:

First I did

console.log("5" + 1);

This prints 51, this is normal right, both number and string have a + operator, but since string is the first variable it will convert 1 to a string.

Now when I did this:

console.log(1 + "5")

I expected output to be 6, as I thought it would convert string to a number. However, the magic output was 15.

Could anyone more experienced in javascript brighten this up for me?

An addition (even of different types) that follows the [commutative law](http://www.mathsisfun.com/definitions/commutative-law.html) isn't a bad thing in general ;-) — linqu, Sep 25 '14 at 12:00
possible duplicate: http://stackoverflow.com/questions/5961000/javascript-sign-concatenates-instead-of-giving-sum-of-variables (first one found, I'm sure there are many out there) — rlemon, Sep 25 '14 at 12:07
@linqu Expect that it is *not* commutative because `"5" + 1 == "51" != "15" == 1 + "5"`. — Bakuriu, Sep 25 '14 at 16:28
It makes more sense than [some other uses of the `+` operator in JavaScript](http://destroyallsoftware.com/talks/wat). — rickster, Sep 26 '14 at 05:29
@Bakuriu good point! I was referring to the data type that does not change. But it's true, result is changing — linqu, Sep 26 '14 at 10:12
It gets concatenated no matter what, as long as there's a string parameter. — rageandqq, Dec 21 '14 at 20:37

score 27 · Accepted Answer · answered Sep 25 '14 at 11:58

27

Quoting ECMAScript spec The Addition operator ( + ) section:

If Type(lprim) is String or Type(rprim) is String, then
Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

So the order doesn't matter here.

answered Sep 25 '14 at 11:58

dfsq

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Beat me to it...The general rule is: If you add a number and a string, the result will be a string! – Sarima Sep 25 '14 at 11:59
Aye, an odd rule though – Robin-Hoodie Sep 25 '14 at 12:00
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Why odd? If there is a string, then the result is going to be string as well. Easy to remember. It would be weird if result depended on the order of operands, I think. – dfsq Sep 25 '14 at 12:01
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Unfortunately, when an operator can be reused the language designers have to chose a rule for each case. IMO this is better than `2 + "Hello"` becoming `2 + the ascii value for "Hello"`. – Sarima Sep 25 '14 at 12:03
It seems more logical to me to be honest, why would String take precedence over number? – Robin-Hoodie Sep 25 '14 at 12:03
Well, to me more logical would be `NaN`. – dfsq Sep 25 '14 at 12:04
@RobinHellemans: When it comes to default operator precedence, apart from those that comes naturally from mathematics, you can't reason about their behavior. In these cases, the specification simply enforces the behavior. – nhahtdh Sep 25 '14 at 15:26
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@Joshpbarron: What the heck is "the ASCII value for 'Hello'"?? – Lightness Races in Orbit Sep 25 '14 at 15:30
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072 101 108 108 111 @LightnessRacesinOrbit – Sarima Sep 25 '14 at 16:06
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@LightnessRacesinOrbit: I'm sure there's a way in PHP to make `2 + "Hello" = 2 + the ascii value for "Hello".` Have to look it up. :P – sampathsris Sep 25 '14 at 16:18
Again, there is no ASCII value for "Hello". There is a value for each of its constituent characters, which is not the same thing. – Lightness Races in Orbit Sep 25 '14 at 16:19
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@LightnessRacesinOrbit, for the sake of 1,000,000 comments, you are correct. – Sarima Sep 25 '14 at 18:34

score 8 · Answer 2 · answered Sep 25 '14 at 18:17

console.log(1 + "5") I expected output to be 6, as I thought it would convert string to a number. ...

But then what would you expect if you had written the following?

console.log(1 + " fine day")

or

console.log(1 + " answer(s) to my question")

There can be no assurance as a general rule that a string is convertible to a number. But any number can be converted to a string. That's why the conversion rules are written to move toward a type that is compatible. (In contexts where you as a programmer know that a string can be safely converted to a number, then you can do so explicitly so that the + operation is between two numbers. But that is not true in general for strings.)

In other contexts, this is also why small ints and low precision floats would be converted to large ints or double precision floats when operating on mixed types that the latter types. You can safely convert the limited forms into the larger forms, but you cannot safely go in the other direction in general. A small int can be represented as a big int or a double, but the other direction is not generally a safe conversion.

So conversion rules for operation on mixed types are written as much as is feasible to move toward mutually compatible types that are safe common types. It is left to the programmer to write explicit conversions for the special cases where a more general type can be safely converted to a more limited type.

score 2 · Answer 3 · edited Sep 25 '14 at 15:19

2

In both cases the value will be converted to a string. When you add a number to a string or a string to a number, the result is a string.

Simply use parseInt(value) or toString(value) to force the conversion.

edited Sep 25 '14 at 15:19

nhahtdh

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answered Sep 25 '14 at 12:05

Bernd

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Good answer except no citations! – Lightness Races in Orbit Sep 25 '14 at 15:30

score 1 · Answer 4 · answered Sep 26 '14 at 04:08

1

You can try code :
console.log(1 + parseInt("5")) => 6

answered Sep 26 '14 at 04:08

Ryan Nghiem

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score -6 · Answer 5 · answered Sep 25 '14 at 12:05

-6

Concatenation also uses + in Javascript.

var result = "String" + number + obj;
// is equivalent to
string result = "String" + number.ToString() + obj.ToString();

However thing is, in C# / .net you could do the same thing, and you will get the same result - System.Console.WriteLine("result is " + 10 + 20);.

In JavaScript (and C# for that matter) strings are immutable. They can never be changed, only replaced with other strings. You're probably aware that combined + "String" doesn't directly modify the combined variable - the operation creates a new string that is the result of concatenating the two strings together, but you must then assign that new string to the combined variable if you want it to be changed.

The argument about using mathematical operators to do string concatenation is arguably an "incorrect" argument, but there's also an argument to be made that using + to do a lot of string concatenation can be very slow.

answered Sep 25 '14 at 12:05

Nadeem_MK

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"but there's also an argument to be made that using + to do a lot of string concatenation can be very slow." this isn't very true anymore. http://jsperf.com/string-concat-without-sringbuilder/5 which I got linked from [this answer](http://stackoverflow.com/a/4717855/829835) – rlemon Sep 25 '14 at 14:38
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How does this even answer the question? – nhahtdh Sep 25 '14 at 15:18
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I don't understand why so much downvote, was just giving a simple explanation to OP on his/her question!!! What the f*ck!! – Nadeem_MK Sep 26 '14 at 06:08
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the comment from @nhahtdh explains the downvotes. You're not answering the question. The question is _not_ about using mathematical operators for string concatenation like you assumed. – Darko Sep 26 '14 at 06:39
If that's the case, how come OP selected it as answer in the 1st place? only when you started downvoting that he changed the answer!! – Nadeem_MK Sep 26 '14 at 08:23

Why is number + string a string in javascript?

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