What are the top JavaScript pitfalls? [closed]

Question

Boolean type conversion.

''        ==   '0'           //false
0         ==   ''            //true
0         ==   '0'           //true
false     ==   'false'       //false
false     ==   '0'           //true
false     ==   undefined     //false
false     ==   null          //false
null      ==   undefined     //true
" \t\r\n" ==   0             //true

As well as the difference between null and undefined. As listed in the table above, comparing null & undefined with == returns true, but with === it returns false. This behavior makes sense once you understand that undefined is very different from a variable having a null value, and something holding the value undefined is different from something being undefined.

Don't accidentally leave a trailing comma in an object definition literal or IE will fail and you won't notice until much later because you never use IE for development and by then it could suck figuring out what happened.

var foo = { 
    bar: "bar", 
    baz: "baz", 
};

Note @JulianR's comment: In arrays, IE doesn't fail directly by throwing some syntax error, but will fail when you try to use the array because the added comma makes IE think there's one more element in the array, with value undefined, than there actually is. So if you ever have an error because for some reason the last element in an array is undefined: it's a comma.

Admittedly I've been guilty of some of these in the past, for your amusement it's the ones in bold:

• Not knowing incorrect (and the very few correct) uses of eval

  eval("obj."+prop);

• Using with statements
• Using parseInt(str, base) without specifying the base argument.
• Using this in timer/callback functions.
• Using eval-like expressions in timers

  setTimeout("someFunc(myScopedVarWhoops)");

• Thinking jQuery is the name of the language you're coding
• Performing simple JavaScript tasks using a framework -- $(1).plus(1) anyone? ;-)
• Using continue without incrementing or adjusting the conditional variable.
• Flooding the global namespace with variables
• Forgetting var in or before for statements. for (i=0;i<10;i++)
• Using an obfuscator and just letting it run wild on your code
• Not really a pitfall, but pointless - return condition ? true : false; instead of return condition;
• Not commenting your code, applies to all languages really.
• Using try...catch...finally statements to catch errors instead of using if statements to check variables.
• Foolishly attempting to stop "view source" by blocking right mouse clicks on your pages (I was young *sobs*!)
• Using { 0: "Foo", 1:"Bar", 2:"Foobar" } instead of [ "Foo", "Bar", "Foobar" ]
• Using parseInt() on user input

  parseInt("1,000") // -> 1, wrong!
  +"1,000" // -> NaN, correct!

Some already mentioned:

• Not using strict equality (===) operators whenever possible
• Setting event handlers to the return value of a function instead of a reference to said function
• Not ; terminating statements properly
• Using for...in loops on arrays

Might think of some more after I've slept :-)

• Forgetting to declare variables with var
• Misunderstanding (or not understanding) variable scope and closures
• Trying to solve nasty compatibility problems that framework teams have already solved