Use the d8 shell to run the v8 engine. Use the following pattern to run on a file:

/path/to/d8 [flags] [file].js

For example:

./d8 --log-gc script.js

will run d8 on script.js with garbage collection logging enabled

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.

Example:

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

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

load("external.js");

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

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

quit(0); // bye

Worker example:

main.js

var workerScript = read("worker.js");
var worker = new Worker(workerScript);
worker.postMessage(12);

worker.js

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);
childGlobal.x++;
Realm.eval(rIndex3, "print(x)"); // 51

try {
  Realm.dispose(rIndex3);
  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)");