v8 Useful built-in functions and objects in d8


In addition to libraries defined in EcmaScript language specification and EcmaScript internationalization API specification, d8 also implements the following functions and objects.

  • print(args...): function. Print to stdout.
  • printErr(args...): function. Print to stderr.
  • write(args...): function. Same as print but no newline at the end.
  • read(filename): function. Read text from file and returned as String.
  • readbuffer(filename): function. Read binary from file and returned as ArrayBuffer.
  • readline(): function. Read line from stdin. Use '\' for multi-lines input.
  • load(filename): function. Load and execute JavaScript file.
  • quit([exitCode]): function. Quit with optional exit code.
  • version(): function. Return version code as String.
  • os: object. OS-related utilities, only available for POSIX.
    • os.system(command): function: Execute system command.
    • os.chdir(path): function: Change current directory.
    • os.setenv(name, value): function: Set environment variable.
    • os.unsetenv(name): function: Unset environment variable.
    • os.umask(alue) function: Calls the umask system call and returns the old umask.
    • os.mkdirp(path[, mask]): function: Creates a directory. The mask (if present) is anded with the current umask. Intermediate directories are created if necessary.
    • os.rmdir(path): function: Remove directory.
  • performance: object: Use for performance analysis.
    • performance.now(): function. Returns a time stamp as double, measured in milliseconds.
  • Worker: object: Modified from HTML5 Web Worker (see example below for detail)
  • Realm: object: Create and manage isolated environment (realm).
    • Realm.create(): function: Create a new realm with distinct security token and return its index.
    • Realm.createAllowCrossRealmAccess(): function: Create a new realm with same security token as the current realm and return its index.
    • Realm.current(): function: Returns the index of the currently active realm. Index of global realm is 0.
    • Realm.global(i): function: Returns the global object of realm i.
    • Realm.owner(globalObj): function: Returns the index of the realm that created globalObj.
    • Realm.eval(i, s): function: Evaluates s in realm i and returns the result.
    • Realm.switch(i): function: Switches to the realm i for consecutive interactive inputs.
    • Realm.dispose(i): function: Disposes the reference to the realm i.
    • Realm.shared: object: An accessor for a single shared value across realms.

Detailed implementations and comments can be found in d8.h, d8.cc, d8-posix.cc and d8-windows.cc.


print("Hello World!");
write("Hello ");
printErr("Nothing went wrong.");

write("Your name: ");
var name = readline();
print("Hello, ", name, "!");


var string = read("text.txt");
var buffer = readbuffer("binary.bin");

print("Version: ", version());

quit(0); // bye

Worker example:


var workerScript = read("worker.js");
var worker = new Worker(workerScript);


onmessage = ev => {
  print(ev); // 12

Realm example:

print(Realm.current()); // 0

var rIndex = Realm.create();
print(rIndex); // 1

Realm.eval(rIndex, "var x = 100");
Realm.eval(rIndex, "print(x)"); // 100
var result = Realm.eval(rIndex, "x * 2");
print(result); // 200

Realm.eval(rIndex, "var rIndex2 = Realm.create()");
Realm.eval(rIndex, "print(rIndex2)"); // 2
Realm.eval(rIndex, "print(Realm.owner(this))"); // 1

try {
  var childGlobal = Realm.global(rIndex);
  print(childGlobal); // error
} catch (e) {
  print("Global object cannot be read/written cross-realm.");

var rIndex3 = Realm.createAllowCrossRealmAccess();
Realm.eval(rIndex, "var x = 50");
var childGlobal = Realm.global(rIndex3);
Realm.eval(rIndex3, "print(x)"); // 51

try {
  Realm.eval(rIndex3, "print(x)"); // error
} catch (e) {
  print("The realm is dereferenced");

Realm.shared = "Hello from another world";
Realm.eval(rIndex, "print(Realm.shared)");