bizarre javascript arithmetic behavior (yup... to be expected)

Question

ok, I'm writing a little code snippet to get the ISO date-format value for yesterday.

code:

var dateString = new Date();

var yesterday = dateString.getFullYear();

    yesterday += "-"+dateString.getMonth()+1;

    yesterday += "-"+dateString.getDate()-1;

The above code outputs 2009-111-23. It is clearly not treating dateString.getMonth() as an intiger and tacking 1 on to the end of it.

Does putting the "-"+ in front of dateString.getDate() cast getDate() into a string?

this works gets the desired result.

var dateString = new Date();

var yesterday = dateString.getFullYear() + "-";

    yesterday += dateString.getMonth()+1+ "-";

    yesterday += dateString.getDate()-1;
//yesterday = 2009-12-22

Although I don't really like the way it looks... whatever no big deal.

Can anyone explain to me why javascript acts like this? is there any explanation for why this happens?

Answer 1

This is about associativity. + operator is left-associative, so

"-"+dateString.getMonth() + 1

is same as

("-"+dateString.getMonth()) + 1

Put parenthesis around the expression you want to be evaluated first:

"-" + (dateString.getMonth() + 1)

Answer 2

The correct way to get a date value representing "yesterday" is this:

var today = new Date();
var yesterday = new Date(today.getTime() - (1000*60*60*24));

From there you can get the values of interest, like yesterday.getDate(),.