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up vote 3 down vote favorite

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?

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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 Apr 24 at 17:09

1 Answer 1

up vote 5 down vote accepted

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.

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1  
Thanks.. I was always under the impression that x++ and x += 1 were the same but apparently this is an important difference. – billynoah Apr 24 at 17:03
    
@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. :-) – T.J. Crowder Apr 24 at 17:11

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