According to ECMA-262, part 11.13, following is the exhaustive list of compound assignment operators: *= /= %= += -= <<= >>= >>>= &= ^= |=.
According to the part 11.11, var c = a || b will put a value into c if ToBoolean(a) is true and will put b value into c otherwise. As such, the logical OR is often used as the coalesce operator, e.g.
function (options) {
options = options || {};
}
Frequently enough, coalesce is used to specify the default value for the variable, as was shown above: a = a || b.
It seems that compound assignment operator ||= would be really useful, allowing to write the code above in a shorter and cleaner fashion: a ||= b. However, it is not there (although *=, += and other compound assignment operators are).
The question is, why?
7
One possible reason is that the logical operators && and || have “short-circuiting” behavior. The right-hand operand of && and || is not evaluated unless necessary. Perhaps for this reason the language designers decided that the meaning of an expression like a ||= f() was not obvious, and so such operators were better left out.
2
The general answer to all questions about “why was this language feature not implemented” is that the team who designed the language decided that the benefit didn’t outweigh the cost.
Cost can take many forms. It takes time and effort to implement a language feature, but there is also the intrinsic cost of complexity: does the feature make the language more complex or ambiguous, out of proportion to its potential benefit?
The hypothetical ||= operator does something fundamentally different from the other compound assignment operators. While the other operators are purely mathematical in nature, this one is different: it substitutes one value for another (in the context you described).
Given this ambiguity (the operator performs two different functions, depending on context), it’s not difficult to see why it was not included in the language. Although you state that changing
function (options) {
options = options || {};
}
to
function (options) {
options ||= {};
}
to perform null coalescing is a valuable feature, the benefit is far less clear to me. If value substitution is to occur, it seems logical (and clearer) to have both values on the right side of the equals sign, to provide a visual indication that such substitution may occur.
C# took a different path, and uses a specific operator for null coalescing.
6
You are right that ||= is a useful construct. It exists in Perl.
In fact Perl makes all of these available:
**= += *= &= <<= &&= -= /=
|= >>= ||= .= %= ^= //= x=
Some of these are great (.= adds something to the end of a string), others less so (&&=??? I suppose the variable would get set to the right side if both it and the variable are true. But why would you ever do this?)
What is included in a language is really a feature of its design philosophy.
1
The question is, why?
Because it was not designed and implemented yet.
As of ECMAScript 2021 ||= is a valid assignment alongside =, **=, *=, /=, %=, +=, -=, <<=, >>=, >>>=, &=, ^=, |=, &&=, and ??=.
The omission of the logical assignments was not because somebody sat down and said “We need these ones but will most definitely not be including those ones and This Is How The Language Is To Be”. Rather, it takes time and effort to add each language feature. If that time and effort is not put in or not enough, you do not get the language feature.
The explanation seems underwhelming but it is what it is. Programming languages are most often held back by human limitations to how much time and effort they have, rather than the humans deliberately imposing limitation on what a programming language should be able to do.