The woes of trailing commas in IE

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If you leave a trailing comma in your JavaScript code, you’re a terrible person.
It’s right up there with omitting semicolons
(https://github.com/twitter/bootstrap/issues/3057) as far as I’m concerned.

While fixing some issues on a site in IE7 recently, I was greeted by the
familiarly unhelpful error message from Internet Explorer 7 that prompted me to
write this.

Error: Expected identifier, string or number

You can see an example below:

[IMAGE: https://blog.omgmog.net/images/by-default-2012-11-20-at-16.27.28.png]

What does that tell us? Bugger all in fact, unless you’ve tackled this problem
before. It’s really unhelpful, but it’s caused by having a trailing comma inside
an array/object/method, like the following:

forms.set_errors(
    $form,
    response.form_errors || [],
    response.field_errors,
);

That right there, that’s not cool. It’ll make Internet Explorer explode.

If you had any consideration you would write something more like:

forms.set_errors(
    $form,
    response.form_errors || [],
    response.field_errors
);

tasty.

You can prevent this problem occuring by using a linter
(http://www.jslint.com/), or you can retroactively go and find the
problem-causing lines in your JavaScript using a RegEx pattern such as the
following:

,[\s\n]*[^\[\{\w\n\s/\*\"\'\$\#\.\`\:\|\!]

Though this is valid according to ECMAScript 5, it’s another one of those bodged
features in Internet Explorer that happens to behave unlike every other browser,
so I think it’s best to go for the solution that works for everybody.

That’s about all I’ve got to say, I just wanted to rant about this and how
terrible IE7 is.

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Published November 20, 2012

Generated from the original post:
https://blog.omgmog.net/post/the-woes-of-trailing-commas-in-ie/


Max Glenister is an interface designer and senior full-stack developer from
Oxfordshire. He writes mostly about front-end development and technology.

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