D8 (Debug8) is V8's own minimalist debug shell.

D8 is useful for running some JavaScript locally or debugging changes you have made to V8. A normal V8 build using [[GN|Building with GN]] for x64 will output 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 is very simple with the print() function:

--- test.js ---
print("Hello, World!");
out.gn/x64.optdebug/d8 test.js
> Hello, World!

Read Input

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

d8> var 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
... (etc.) "

Use readline() to interactively enter text:

d8> var greeting = readline();
d8> greeting

Load External Scripts

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

--- util.js ---
function greet(name) {
  return "Hello, " + name;
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 and you will be able to 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]

More Resources

Kevin Ennis' D8 Guide has really good information about exploring V8 using D8.