set variable inside val

Question

This appears to work and is valid, but is there any reason I shouldn't do this? It saves me a line of code and lets me set a variable and the value of a text area.

$('#price').val(default_price = 2.9);

It's equivalent to this:

default_price = 2.9;
$('#price').val(default_price);

Answer 1

It embeds code doing one thing inside code doing something entirely different.

Particularly if you're talking about default values, "constants", etc., conflating initialization with UI interaction leads to confusion. Keep them separate–easier to find and maintain.

Technically it's the same thing. Cognitively it isn't.

o.v. raises the specter of global namespace pollution. By declaring variables in arbitrary locations you increase the odds of over-writing a value, fat-fingering an identifier, duplicating effort, and so on.

In addition to creating difficult-to-isolate bugs, this is an additional cognitive load, because you then have to understand the scope of the declared variable, locate where else it may be used, etc.

Answer 2

I'll entertain the idea such a construct could be acceptable in certain cases but not this one especially since there are other stylistic issues in the given example (the biggest one being "where does the magic number come from")

Main concern IMO is whether or not the variable has been declared - you can't simply

$('#price').val(var default_price = 2.9); //nope

and if the original code is used with the variable not yet declared, you end up polluting the global scope. However, if the variable has been declared, it brings up a follow-up question "why has it not been declared with the correct default value". Alternatively, the magic number could be different depending on an (unknown) condition:

if (/*whatever*/) {
  $('#price').val(default_price = 2.9);
} else {
  $('#price').val(default_price = 9522); //over 9000
}

Again, this is stylistically poor since setting the value of #price should have been executed outside of the conditional (or a switch statment):

if (/*whatever*/) {
  default_price = 2.9;
} else {
  default_price = 9522;
}
$('#price').val(default_price);

There could be a convoluted case where the variable setter is overridden to return something other than the value assigned which is a bit of a questionable practice in the first place IMO.