javascript - How can % signs be used in identifiers

Question

Browsing the v8 tree, under the src directory, some js files were there, providing some basic JS objects like Math, Array etc. Browsing those files, I saw identifiers including a percent sign (%) in their names, i.e. %Foo. I first naively thought it was some other allowed character in JS's identifiers, but when I tried it in shell, it yelled at me, saying that I'm violating syntax rules. But if it is a syntax error, how come d8 works? Here are an example from the actual source code:

src/apinatives.js lines 44 to 47, git clone from github/v8/v8

function Instantiate(data, name) {
  if (!%IsTemplate(data)) return data;
      var tag = %GetTemplateField(data, kApiTagOffset);
      switch (tag) {

src/apinatives.js lines 41 to 43, git clone from github/v8/v8

function SetConstructor() {
  if (%_IsConstructCall()) {
    %SetInitialize(this);

How come this identifiers do not yield syntax errors. All js files, including math.js and string.js and all others?:wq

Answer 1

It is not technically valid JavaScript. These are calls to V8 runtime functions. From that page:

Much of the JavaScript library is implemented in JavaScript code itself, using a minimal set of C++ runtime functions callable from JavaScript. Some of these are called using names that start with %, and using the flag "--allow-natives-syntax". Others are only called by code generated by the code generators, and are not visible in JS, even using the % syntax.

If you look in parser.cc you can see some code relating to allow_natives_syntax that determines whether the parser will accept this extension to the JavaScript language that V8 is using to interact with its runtime. These files must be parsed with that option enabled.

I would speculate that V8 does not allow you to make these calls by default both because it would contradict the JavaScript standard and because it would probably allow you to do things to the runtime you should not be able to do.