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EcmaScript Spec

To further my understanding of the V8 code base I've found it helpful to read the spec. This can make it clear why functions and fields are named in certain ways and also why they do certain things.

[[something]]

Most often denotes an internal property named something.

@@

Within the specification a well-known symbol is referred to by using a notation of the form @@name, where name is one of the values:

Spec Name               [[Description]]
* @@asyncIterator       Symbol.asyncIterator
* @@hasInstance         Symbol.hasInstance
* @@isConcatSpreadable  Symbol.isConcatSpreadable
* @@iterator            Symbol.iterator
* @@match               Symbol.match
* @@matchAll            Symbol.matchAll
* @@replace             Symbol.replace
* @@search              Symbol.search
* @@speices             Symbol.species
* @@split		Symbol.split
* @@toPrimitive		Symbol.toPrimitive
* @@toStringTag         Symbol.toStringTag

So you might see something like [@@toPrimitive] anobject would have a function keyed with Symbol.toPrimitive which can be used access that function.

Record

A Record in the spec is like a struct in c where each member is called a field.

Completion Record

Is a Record which is used as a return value and can have one of three possible fields:

[[Type]] (normal, return, throw, break, or continue)

If the type is normal, return, or throw then the CompletionRecord can have a

[[Value]] which is what is returned/thrown.

[[Target]] If the type is break or continue it can optionally have a [[Target]].

For example:

function something() {
  if (bla) {
    return CompletionRecord({type: "normal", value: "something"});
  } else {
    return CompletionRecord({type: "throw", value: "error"});
  }
}
const result = something();

So a function in the spec would return a CompletionRecord and the spec has to describe what is done in each case.

ReturnIfAbrupt

If it was an abrupt (throw) in the example above it return the value. Instead of writing that as text the spec writers can use:

ReturnIfAbrupt(something())

can be written as:

? something()

ReturnIfAbrupt means if the value passed in is an abrupt completion then that value is returned, else if the value is a Completion Record then set the value to value.[[Value]].

function something() {
  return CompletionRecord({type: "normal", value: "something"});
}
const result = something();
Let result be !something()

This means that something() will never return an abrupt completion.

Every abstract operation in the spec implicitely returns a completion record.

Sets

You can find sets uses in the spec in places where you have something like << one, two >> which denotes a set with two elements.

Builtin Objects

In the spec one can see object referred to as %Array% which referrs to builtin objects. These are listed in well-known-intrinsic-objects.

Realms

All js code must be associated with a realm before it is evaluated which consists of the builtin/intrinsic objects, a global environment (link. This could be a window/frame/tab in a browser, another example could be a web worker or a service worker. Code in one realm might not be able to interact with code in another. Like a symbol might only be available in one for example. TODO: add more examples and clarify this.