Jason Orendorff from Mozilla published a great in-depth analysis of JS syntactic quirks. Even though the implementation details differ, every JS engine faces the same problems with these quirks.
In this episode, we’ll go deeper in the definition of the ECMAScript language and its syntax. If you’re not familiar with context-free grammars, now is a good time to check out the basics, since the spec uses context-free grammars to define the language. See the chapter about context free grammars in "Crafting Interpreters" for an approachable introduction or the Wikipedia page for a more mathematical definition.
Let’s practice our awesome spec reading skills some more. If you haven’t had a look at the previous episode, now it’s a good time to do so!
A fun way to get to know the spec is to start with a JavaScript feature we know is there, and find out how it’s specified.
Warning! This episode contains copy-pasted algorithms from the ECMAScript spec as of February 2020. They’ll eventually be out of date.
We know that properties are looked up in the prototype chain: if an object doesn’t have the property we’re trying to read, we walk up the prototype chain until we find it (or find an object which no longer has a prototype).
For example:
const o1 = { foo: 99 };
const o2 = {};
Object.setPrototypeOf(o2, o1);
o2.foo;
// → 99
Let’s try to find out where this behavior is defined. A good place to start is a list of Object Internal Methods.
There’s both [[GetOwnProperty]] and [[Get]] — we’re interested in the version that isn’t restricted to own properties, so we’ll go with [[Get]].
Unfortunately, the Property Descriptor specification type also has a field called [[Get]], so while browsing the spec for [[Get]], we need to carefully distinguish between the two independent usages.
In this article, we take a simple function in the spec and try to understand the notation. Let’s go!
Even if you know JavaScript, reading its language specification, ECMAScript Language specification, or the ECMAScript spec for short, can be pretty daunting. At least that’s how I felt when I started reading it for the first time.