Possible cases for Javascript error: "Expected identifier, string or number"

Question

Some users are reporting occasional JS errors on my site. The error message says "Expected identifier, string or number" and the line number is 423725915, which is just an arbitrary number and changes for each report when this occurs. This mostly happens with IE7/ Mozilla 4.0 browsers.

I scanned my code a bunch of times and ran jslint but it didn't pick anything up - anyone know of the general type of JS problems that lead to this error message?

Answer 1

The cause of this type of error can often be a misplaced comma in an object or array definition:

var obj = {    id: 23,    name: "test",  <-- } 

If it appears at a random line, maybe it's part of an object defintion you are creating dynamically.

Answer 2

Using the word class as a key in a Javascript dictionary can also trigger the dreaded "Expected identifier, string or number" error because class is a reserved keyword in Internet Explorer.

BAD

{ class : 'overlay'} // ERROR: Expected identifier, string or number 

GOOD

{'class': 'overlay'} 

When using a reserved keyword as a key in a Javascript dictionary, enclose the key in quotes.

Hope this hint saves you a day of debugging hell.