Navigating the ECMAScript® 2015 Language Specification
Here is one I encountered a few minutes ago on IRC:
why can't we access an array property as
Well, explaining the reason why — the design decisions which led to this "limitation" is not so easy. Finding what the specification says, on the other hand, is not so hard.
We'll start here: https://tc39.github.io/ecma262/ and we're looking for the part which talks about accessing properties (accessing any properties, in fact).
If you don't know where to look, quickly find the corresponding MDN page and see that it's called a "property accessor" and that it's a left-hand-side expression.
Back to the ECMAScript specification, find the index at the beginning of the page.
12.3 Left-Hand-Side Expressions, good.
12.3.2 Property Accessors, perfect!
Now, we want to know what can be used as
name in the following syntax:
object.name, right? It's pretty clear:
MemberExpression . IdentifierName.
But what is an
IdentifierName? A quick search in the page brings us to the following section:
11.6.1 Identifier Names.
IdentifierName :: IdentifierStart IdentifierName IdentifierPart
We now know that an
IdentifierName has to start with an
IdentifierStart, let's look for
IdentifierStart in the page...
IdentifierStart :: UnicodeIDStart $ _ \ UnicodeEscapeSequence [...] UnicodeIDStart :: any Unicode code point with the Unicode property “ID_Start”
IdentifierStart is either a Unicode code point with the Unicode property "ID_Start" or
_ or a Unicode escape sequence.
Back to the original question: why can't we do
'0' is our
IdentifierName and its
0 is not
_ or a Unicode escape sequence, it has to be a
UnicodeIDStart to be a valid property accessor when used with the dot notation.
0 is the Unicode code point
U+0030. Let's see what's in there using codepoints.net:
ID Start? ✘
There it is.
0 does not have the property "ID_Start". No ID_Start ⇒ not a valid
IdentifierStart ⇒ not a valid
IdentifierName ⇒ not a valid property accessor using the dot notation.
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