Using `d8`

d8 is V8’s own developer shell.

d8 is useful for running some JavaScript locally or debugging changes you have made to V8. Building V8 using GN for x64 outputs a d8 binary in out.gn/x64.optdebug/d8. You can call d8 with the --help argument for more information about usage and flags.

Print to the command line #

Printing output is probably going to be very important if you plan to use d8 to run JavaScript files rather than interactively. This can be achieved using console.log:

$ cat test.js
console.log('Hello world!');

$ out.gn/x64.optdebug/d8 test.js
Hello world!

d8 also comes with a global print function that does the same thing. However, console.log is preferred over print since it works in web browsers as well.

Read input #

Using read() you can store the contents of a file into a variable.

d8> const license = read('LICENSE');
d8> license
"This license applies to all parts of V8 that are not externally
maintained libraries.  The externally maintained libraries used by V8
are:
… (etc.)"

Use readline() to interactively enter text:

d8> const greeting = readline();
Welcome
d8> greeting
"Welcome"

Load external scripts #

load() runs another JavaScript file in the current context, meaning that you can then access anything declared in that file.

$ cat util.js
function greet(name) {
  return 'Hello, ' + name;
}

$ d8
d8> load('util.js');
d8> greet('World!');
"Hello, World!"

Pass flags into JavaScript #

It’s possible to make command-line arguments available to your JavaScript code at runtime with d8. Just pass them after -- on the command line. You can then access them at the top-level of your script using the arguments object.

out.gn/x64.optdebug/d8 -- hi

You can now access an array of the arguments using the arguments object:

d8> arguments[0]
"hi"

More resources #

Kevin Ennis’s D8 Guide has really good information about exploring V8 using d8.

Background of the name d8: very early on, V8 had a “sample shell”, whose purpose was to demonstrate how V8 could be embedded to build a JavaScript shell. It was intentionally minimalist, and was simply called “shell”. Soon after, a “developer shell” was added with more convenience features to aid developers in their daily work, and it needed a name too. The original reason why “d8” was chosen as a name is lost to history; it caught on because “eveloper” is 8 omitted characters, so “d8 shell” makes sense as an abbreviation, and also fits in nicely with “V8” as the project name.

Using d8