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JavaScript Idiosyncrasies

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

Also, see my previous posts on PCjs Coding Conventions and JavaScript Negativity.

Strict Equality

Many JavaScript websites 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), and the use of “non-strict” operators, 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). It is shorthand for:

if (b == false) { ... }

yet I suspect the proponents of strict equality would embrace the former while rejecting the latter.

However, I don’t recommend “falsy” checks for optional parameters:

if (!parameter) { ... }

because often a valid numeric parameter might include 0, or a valid string parameter might include “”, so it’s better to do this:

if (parameter == null) { ... }

and obviously if null is also a acceptable value, then you should definitely use strict equality:

if (parameter === undefined) { ... }

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
• Use strict equality operators (just don’t use them mindlessly)

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;

The same problem arises with objects using numeric properties. And watch out for JavaScript’s automatic base conversion of numeric properties. For example, when you enumerate the properties of object “o”:

var o = {
    0x20: ' ',
    0x41: 'A'
};

you will get the strings “32” and “65”, not “0x20” and “0x41”. You must quote your property names to prevent any conversion; eg:

var o = {
    "0x20": ' ',
    "0x41": 'A'
};

Numeric properties can always be safely converted using the unary “+” operator, regardless whether they were quoted or not.

The unary “+” is a great alternative to parseInt(), but be mindful of their differences. One important difference is that parseInt() will stop when it encounters an invalid digit, returning whatever value was parsed up to that point, whereas unary “+” conversion will return NaN if there are any invalid digits in the string.

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 32-bit 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.

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

Jeff Parsons
Mar 26, 2015