Why does increment using ++ work with strings in javascript?

Question

Normally when I get the value of a form input with jQuery I am obliged to convert it to a number before performing any mathematical operations. For instance, using the unary plus operator to convert, and then increment:

var x = +$(this).val();
x += 1;

But for some reason ++ works with strings and does the conversion automatically:

var x = $(this).val();
x++;

http://jsfiddle.net/m4nka9d3/

Why? Is this always safe?

One of the most interesting questions I've seen in weeks. And which, like most really good questions, taught me something in the process of answering. — T.J. Crowder
– T.J. Crowder, Commented Apr 24, 2015 at 17:09

T.J. Crowder · Accepted Answer · 2015-04-24 17:09:38Z

5

It works that way because the first thing either of the increment operators does is coerce its operand to a number, this is in §11.3.1 of the spec. And being in the spec, yes, it's something you can rely on.

Skipping some spectalk, the algorithm is:

Let oldValue be ToNumber(GetValue(lhs)).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see 11.6.3).
Call PutValue(lhs, newValue).
Return oldValue.

Whereas, += is defined quite differently, not coercing to number as an early step, and thus ending up applying string concatenation rather than addition.

edited Apr 24, 2015 at 17:09

answered Apr 24, 2015 at 17:02

T.J. Crowder

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2 Comments

You Old Fool Over a year ago

Thanks.. I was always under the impression that x++ and x += 1 were the same but apparently this is an important difference.

T.J. Crowder Over a year ago

@billynoah: That's a very, very good observation. In most languages with B-like syntax (C, Java, etc.), ++x being essentially x += 1 (not x++, the final value of the expression is different) would be an accurate statement. But not JavaScript, with its loose typing. :-)

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