JavaScript Idiosyncrasies
---
Time to mention a few JavaScript idiosyncrasies, and how I deal with them.

Also, see my previous posts on [PCjs Coding Conventions](/blog/2014/09/30/) and [JavaScript Negativity](/blog/2014/10/26/).

### Strict Equality

Most sites will advise you to *never* use the "==" and "!=" JavaScript operators, because when they compare variables
containing different data types, JavaScript will coerce one of the operands to a matching type, sometimes in unexpected
ways.  We can thank the early days of JavaScript for this feature, when it was trying to be extraordinarily forgiving
of sloppy code.  I'm not going to list all the odd results that can arise from JavaScript's operand coercion, because
there are more than enough examples on the web already.

To avoid unexpected type coercion, and thus unexpected matches and/or mismatches, the usual advice is to *always* use
strict equality operators ("===" and "!==").

I disagree.

In well-written code, the variable data types should always be clear.  In fact, the more you're able to
use JSDoc types to declare the data types of all your parameters, return values, and other variables, the fewer errors
you'll have.  As long as you're always comparing variables with matching types, there shouldn't be any unexpected
coercions.

Obviously, there will be times when a polymorphic variable is required, especially when dealing with APIs that can
return multiple types.  But those should be the exception, not the rule.

Another exception is optional parameters.  When I write a method with optional parameters, I generally allow those
parameters to either be omitted (ie, *undefined*) or set to *null*.  Using "==", you can check for either value with
a single comparison:

	if (parameter == null) { ... }
	
whereas strict equality requires more work:
 
	if (parameter === undefined || parameter === null) { ... }

This is one of those times when coercion (of *undefined* to *null*) is beneficial.  Here's another:

	if (!b) { ... }

Coercing a value to *boolean* is a popular way of checking for all "falsy" values (ie, *undefined*, *null*,
0, false, "", NaN, etc).  Again, another situation where type coercion is beneficial and well understood.

However, don't use that technique to check for optional parameters:

	if (!parameter) { ... }

if a valid numeric parameter could include 0, or a valid string parameter could include "", etc.

Problems with type coercion are **NOT** problems caused by a poor choice of operators, so trying to make
those problems go away by artificially limiting your choice of operators seems like the wrong solution.
Type coercion problems are, by definition, problems involving mismatched types.  Solutions include:

- Avoid comparing variables of different types; or
- Convert your variables to matching types first; or
- Rely on coercion, but be clear about why and when you're doing it

Explicitly convert variables to a single type whenever possible.  For example, I might define a method
that accepts an optional numeric parameter, with a documented default value when it's omitted.  I think it's
important make that parameter unambiguously numeric as soon as possible; eg:

	/**
	 * foo(n)
	 *
	 * Performs a mathematical operation on n and returns a result.
	 *
	 * @param {number} [n] is an optional parameter (defaults to zero if omitted)
	 * @return {number}
	 */
	function foo(n) {
		n = n || 0;
		...
	}

The expression `n || 0` might seem pointless, because *undefined* and *zero* are equivalent in a "falsy" sense, but
*undefined* is not a number, and there will be fewer problems downstream if you ensure that n is *always* a number.

### Enumerating Array or Object Properties

When using *for*...*in* loops like this:

	var a = [100, 200, 300];
	for (var i in a) { ... }
	
the type of variable *i* will be **string** rather than **number**; that is, it will contain "0", "1" and "2" rather
than 0, 1 and 2.  If you then use *i* to set a matching element in another array, that element will not be stored in
the same (numeric) position as the original array.

One solution is to convert *i* to a **number**:

	parseInt(i, 10);

However, a more elegant solution is to use the unary "+" operator to coerce the **string** to a **number**:

	+i;

### Shift Counts For Bitwise Shifts

It turns out that shifting an integer value by more than 31 bits in either direction may not shift as many bits as
you'd expect.  For example:

	n = 0x10000000;
	n >>>= 33;

will shift n by only *one* bit, not 33 bits, and the result will be 0x08000000, not zero.  This is because,
just like the shift instructions on Intel processors, JavaScript converts the shift count to a *mod 32* value
(in other words, it truncates the shift count to a 5-bit value).

So the above example is equivalent to:

	n >>>= 1;

If you really need larger shift counts to work in a consistent manner, you can perform multiple shifts, where each
shift count is in the range 0-31.  Here's one way to shift a number 33 bits:

	n = (n >>> 31) >>> 2;

Also, it's not quite correct to say that a shift count of zero has *no* effect on a number:

	n = 0x88888888|0;       // n is displayed as -2004318072
	n >>>= 0;               // n is displayed as 2290649224

It's true that the bottom 32 bits of the number were not changed, but a side-effect of the unsigned shift operator
is that all the upper sign bits are stripped from the (64-bit) result.

I consider this an anomaly of JavaScript's bitwise operators, because it breaks the "rule" that bitwise operators
operate *only* on the low 32 bits of a number; there are side-effects on the upper 32 bits as well.

Similarly, as soon as you perform any other bitwise operation on the number, even one that does not modify the low
32 bits, the upper bits will revert to the sign of the lower 32-bit value:

	n |= 0;                 // n is displayed as -2004318072 again

*[@jeffpar](http://twitter.com/jeffpar)*  
*March 26, 2015*