What are the top JavaScript pitfalls? [closed]

Answer 1

JavaScript strings are not byte strings, nor are they even Unicode strings. They are UTF-16 strings. Most characters, including international ones, are a single element in a JavaScript string. But some characters (including most emojis) fall outside the Basic Multilingual Plane and thus appear as two elements in a JavaScript string.

Example:

> "♫".length
1
> "                                                                    

                                                        
            

Answer 2

typeof null is object


>>> var i = 1 + undefined; i;
NaN
>>> var i = 1 + null; i;
1

Answer 3

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.

Answer 4

The biggest difficulties I see for the beginner are understanding execution context (i.e., what "this" means whenever and wherever it is encountered) and the prototype system of inheritance.

Answer 5

• 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

Answer 6

• Using window.onload = init(); instead of window.onload = init;
• Boolean equivalences (as mentioned already)
• Closures within a loop.
• Using for in loop variant for iterating over Arrays.
• Not using ; because it's "optional".
• this (just... in general :))
• Not using var
• Knowing that obj.ref === obj["ref"]