Comments on: Javascript Gotcha: Invalid Assignment Left-Hand Side

By: droope

droope — Wed, 28 Apr 2010 16:46:36 +0000

Gotta love javascript error reporting.

By: Eric Pugh

Eric Pugh — Wed, 21 Apr 2010 17:15:32 +0000

I had a bit of ruby code generating a variable in JS that was failing:

check_var__ = false;

I had spaces in core.solr.name, so I swapped to using dashes. And then had more errors, thank you for this post, it highlighted my problem!

By: magicrebirth

magicrebirth — Wed, 30 Sep 2009 16:51:36 +0000

thanks man that saved me tons of time (and coffees!)

By: josh

josh — Thu, 20 Aug 2009 16:52:21 +0000

It was actually a case of blah.style.background-color=’blah’; for me. if i did background_color, the rest of the js ran ok, but i had to use backgroundColor to actually set and access the element’s color.

By: Zachary

Zachary — Sat, 20 Jun 2009 22:38:44 +0000

Thank you so much! I have spent a couple of days trying to figure out what was wrong with a function I created and out of frustration turned to Google and found this page after a lot of searching. This fixed many of the problems I was having. Thanks again!

By: Art

Art — Thu, 11 Dec 2008 16:51:05 +0000

Did you know that this post comes up #2 in Google for the term “invalid assignment left-hand side”?

Well it does, and I’m really glad because you just saved me some serious time trying to research the error.

Kudos!

By: Jesse Thompson

Jesse Thompson — Sat, 15 Nov 2008 01:38:28 +0000

That’s not so inexplicable, “-” just isn’t a valid character for identifiers in javascript. It’s reserved for uses like the minus sign.

Seriously, how else will Javascript know what you want when you talk about:

this.a-that.b

Are we accessing property b within propterty “a-that”, or are we subtracting this.b from this.a? So, the minus sign just can’t be used in bare identifiers.

It’s true that you can use form names that contain characters that don’t make valid JS identifiers, and in that case the bracket syntax saves the day.

As to why using a _ worked instead, there might be a provision in there somewhere which aliases abc-def and abc_def to mean the same thing. I for one would not count on such a provision however, as [] syntax clarifies your intentions unambiguously.

Mangia.

- – Jesse

By: Owen

Owen — Tue, 19 Aug 2008 13:11:28 +0000

@Mike: Thanks for your comment. Yeah. That would have worked too. Just took me ages to figure out what the problem was!

By: Mike Cromwell

Mike Cromwell — Mon, 18 Aug 2008 19:18:30 +0000

I remember having a similar problem, I solved it by using the elements array of the Form object instead, so your example code would become:

opener.document.forms[0].elements['plugin-media4pic'].value = “avatar.jpg”;

hope this helps