Bug 282130 – Web Inspector: Add eager evaluation in Console

• Status: NEW
• Component: Web Inspector

[novov]

"Eager evaluation" in this case means a little preview of the results of the expression as it is being typed, and an expansion of the autocompletion functionality to suggest results from this evaluation. Both FF and Chrome have this feature; when debugging an app that uses Electron on desktop but WebKit on iOS this is the largest disparity in terms of functionality, as it makes working with complex object structures a lot quicker and easier. 

Internally, both browsers evaluate the code and terminate if it cannot be guaranteed that there are no side effects. This is done by maintaining a list of safe operations (property access, etc) under certain conditions (no proxies, etc), and some allowed builtins:
https://searchfox.org/mozilla-central/source/devtools/server/actors/webconsole/eager-ecma-allowlist.js
https://searchfox.org/mozilla-central/source/devtools/server/actors/webconsole/webidl-pure-allowlist.js
https://source.chromium.org/chromium/chromium/src/+/main:v8/src/debug/debug-evaluate.cc;drc=7804fc4cc9837d70a6f52f7617612884cac261a5;l=537

Firefox uses the developer debugging functionality to step through the code and stop if disallowed code is reached: https://searchfox.org/mozilla-central/source/devtools/server/actors/webconsole/eval-with-debugger.js#434

Chrome implements it differently, implementing it as a feature in V8 that runs the code and throws an exception if disallowed code is reached.

[Radar WebKit Bug Importer]

<rdar://problem/138736709>

[Karl Dubost]

I think there is a bug already for this, but I can't find it right now.