debugger: Make '$$' mean the last value

The debugger already stores the result of each printed expression in
variables named $1, $2, etc., so you can refer to them later. This change
makes the variable '$$' always refer to the last printed value.

Adds a bit of documentation explaining how these variables can be printed,
and adds a test for $$ as well as $1, $2, etc.

Based on a commit from Mozilla's original jorendb:
https://hg.mozilla.org/mozilla-central/rev/fe52996292bb6d1acd51a983f9a7ac6faa7724f6
(The GJS debugger was originally based on jorendb and I was looking to see
if any new improvements had been made since I had forked it)

1 files changed, 5 insertions, 1 deletions

diff --git a/modules/script/_bootstrap/debugger.js b/modules/script/_bootstrap/debugger.js
index ddf13ddc..601035cd 100644
--- a/modules/script/_bootstrap/debugger.js
+++ b/modules/script/_bootstrap/debugger.js
@@ -79,6 +79,7 @@ function debuggeeValueToString(dv, style = {pretty: options.pretty}) {
 function showDebuggeeValue(dv, style = {pretty: options.pretty}) {
     const i = nextDebuggeeValueIndex++;
     debuggeeValues[`$${i}`] = dv;
+    debuggeeValues['$$'] = dv;
     const [brief, full] = debuggeeValueToString(dv, style);
     print(`$${i} = ${brief}`);
     if (full !== undefined)
@@ -263,7 +264,10 @@ printCommand.helpText = `USAGE
 PARAMETER
     · expr: expression to be printed
     · pretty|p: prettify the output
-    · brief|b: brief output`;
+    · brief|b: brief output
+
+expr may also reference the variables $1, $2, ... for already printed
+expressions, or $$ for the most recently printed expression.`;
 
 function keysCommand(rest) {
     return doPrint(`Object.keys(${rest})`);