10 biggest JS pitfalls

Wes Garland wes at page.ca
Sun Dec 30 14:13:39 PST 2012

You did not include variants of

var i, a=[];
for (i=0; i < 10; i++) {
print(a[3]);  /* output is 9 */

in your list.  I see related bugs on a regular basis.

The other place I see regular bugs by intermediate coders is related to
'this' in events; but I'm not sure if we can count that as a JS problem or
a DOM bug (Is that 9c in your list?)  --

function A() {
  this.a = 123;
A.prototype.p = function {alert(this.a)};
window.onclick = new A().p;

IIRC, clicking will alert undefined, as the event handler is invoked with
this as the event. IME, this is not what programmers mean when they write

We can work around it by changing the constructor:
function A() {
  this.a = 123;
  this.p = this.p.bind(this);

but of course now we are over-allocating for no good reason IMO.  Very

#3 on your list is entertaining, I have been told that == is faster than
=== in at least one implementation, because that's what sunspider tests.

On 30 December 2012 16:22, Axel Rauschmayer <axel at rauschma.de> wrote:

> [Starting a new thread, just in case.]
> I made a list of the 10 biggest JS pitfalls and most of them will be gone
> under ES6. Much less to worry about, much easier to learn.
> Could you share your 10-biggest list?
> 1. Both undefined and null [not too much of a problem, easily learned]
> 2. Truthy and falsy values [not pretty, but I’ve grown used to them and
> the convenient but sloppy “has a value” checks via `if`]
> 3. == [easy fix: always use ===]
> 4. Parameter handling [fixed in ES6]
> 5. Array-like objects [not completely fixed (DOM...), but `arguments`
> becomes obsolete in ES6]
> 6. Function-scoped variables [`let` FTW in ES6]
> 7. Accidental sharing of data [for-of will help in ES6]
> 8. Creating sub-constructors is messy [fixed via classes and `super` in
> ES6]
> 9. `this` in non-method functions:
>     9a) Referring to the `this` of a surrounding method,
>     9b) accidentally creating globals by forgetting `new`,
>     9c) using methods as callback functions
>     [(a) and (b) fixed by ES6 arrow functions and ES5 strict mode]
> 10. The for-in loop [simple rule: avoid if you can, already possible in
> ES5]
> Thus: 1-3 won’t go away soon. 4-10 are mostly eliminated by ES6.
> Deliberate omissions:
> - Implicit conversions are messy (and a superset of pitfall #2), but seem
> to surprise people much less than the above items.
> - Modules close another important hole in JavaScript, but I wouldn’t
> consider that hole a pitfall.
> There are more pitfalls, but these seemed the biggest ones.
> --
> Dr. Axel Rauschmayer
> axel at rauschma.de
> home: rauschma.de
> twitter: twitter.com/rauschma
> blog: 2ality.com
> _______________________________________________
> es-discuss mailing list
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Wesley W. Garland
Director, Product Development
PageMail, Inc.
+1 613 542 2787 x 102
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