Get array item by 1-indexed value in another field - json

Question

The Firefox file sessionstore-backups/recovery.jsonlz4 contains information about the current Firefox windows in (compressed) JSON format. This is a simplified format representative of that file:
{
"index": 2,
"entries": [
{
"title": "Foo bar 0",
"url": "https://example.com/bar"
},
{
"title": "Foo bar 1",
"url": "https://example.com/bar"
},
{
"title": "Foo bar 2",
"url": "https://example.com/bar"
}
]
}
The index field represents the currently-displayed history item per tab. Being 1-indexed, in the example above it represents the "Foo bar 1" entry.
How can I use that value in jq to get that tab?
For testing on your own system, the following bash line will get you to this point on most modern Linux systems:
lz4jsoncat ~/.mozilla/firefox/t6e99qbe.default-release/sessionstore-backups/recovery.jsonlz4 | jq '.windows[0].tabs' | jq 'sort_by(.lastAccessed)[-1]'
This returns the most recent tab, but with all history items in the entries array. As the user could have navigated back in the history, we need the index value to determine which entry to return. Piped through gron, it looks like this:
json = {};
json.attributes = {};
json.entries = [];
json.entries[0] = {};
json.entries[0].ID = 504;
json.entries[0].cacheKey = 0;
json.entries[0].docIdentifier = 586;
json.entries[0].docshellUUID = "{b6c1e18a-d0f5-4ba2-996e-ddd9fc28322a}";
json.entries[0].hasUserInteraction = true;
json.entries[0].persist = true;
json.entries[0].principalToInherit_base64 = "eyIxIjp7IjAiOiJtb3otZXh0ZW5zaW9uOi8vOTAzYWU2NDgtMzRlNi00NGQ3LTg0NjYtYzgxMWQyMTg2YjBkLyJ9fQ==";
json.entries[0].resultPrincipalURI = null;
json.entries[0].title = "Tridactyl Top Tips & New Tab Page";
json.entries[0].triggeringPrincipal_base64 = "eyIxIjp7IjAiOiJtb3otZXh0ZW5zaW9uOi8vOTAzYWU2NDgtMzRlNi00NGQ3LTg0NjYtYzgxMWQyMTg2YjBkLyJ9fQ==";
json.entries[0].url = "moz-extension://7ba85bcd-8d63-4e74-a723-e627a31d6274/static/newtab.html";
json.entries[1] = {};
json.entries[1].ID = 506;
json.entries[1].cacheKey = 0;
json.entries[1].docIdentifier = 588;
json.entries[1].docshellUUID = "{b6c1e18a-d0f5-4ba2-996e-ddd9fc28322a}";
json.entries[1].hasUserInteraction = true;
json.entries[1].persist = true;
json.entries[1].referrerInfo = "BBoSnxDOS9qmDeAnom1e0AAAAAAAAAAAwAAAAAAAAEYAAAAAAAEBAAAAAAEA";
json.entries[1].resultPrincipalURI = null;
json.entries[1].title = "Stack Overflow - Where Developers Learn, Share, & Build Careers";
json.entries[1].triggeringPrincipal_base64 = "eyIzIjp7fX0=";
json.entries[1].url = "https://stackoverflow.com/";
json.entries[2] = {};
json.entries[2].ID = 508;
json.entries[2].cacheKey = 0;
json.entries[2].csp = "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";
json.entries[2].docIdentifier = 590;
json.entries[2].docshellUUID = "{b6c1e18a-d0f5-4ba2-996e-ddd9fc28322a}";
json.entries[2].hasUserInteraction = true;
json.entries[2].persist = true;
json.entries[2].principalToInherit_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tLyJ9fQ==";
json.entries[2].referrerInfo = "BBoSnxDOS9qmDeAnom1e0AAAAAAAAAAAwAAAAAAAAEYBAAAAGmh0dHBzOi8vc3RhY2tvdmVyZmxvdy5jb20vAAAACAEBAAAAGmh0dHBzOi8vc3RhY2tvdmVyZmxvdy5jb20vAQE=";
json.entries[2].resultPrincipalURI = null;
json.entries[2].title = "Highest scored questions - Stack Overflow";
json.entries[2].triggeringPrincipal_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tLyJ9fQ==";
json.entries[2].url = "https://stackoverflow.com/questions";
json.entries[3] = {};
json.entries[3].ID = 510;
json.entries[3].cacheKey = 0;
json.entries[3].csp = "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";
json.entries[3].docIdentifier = 592;
json.entries[3].docshellUUID = "{b6c1e18a-d0f5-4ba2-996e-ddd9fc28322a}";
json.entries[3].hasUserInteraction = true;
json.entries[3].persist = true;
json.entries[3].principalToInherit_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tL3F1ZXN0aW9ucyJ9fQ==";
json.entries[3].referrerInfo = "BBoSnxDOS9qmDeAnom1e0AAAAAAAAAAAwAAAAAAAAEYBAAAAI2h0dHBzOi8vc3RhY2tvdmVyZmxvdy5jb20vcXVlc3Rpb25zAAAACAEBAAAAI2h0dHBzOi8vc3RhY2tvdmVyZmxvdy5jb20vcXVlc3Rpb25zAQE=";
json.entries[3].resultPrincipalURI = null;
json.entries[3].title = "Tags - Stack Overflow";
json.entries[3].triggeringPrincipal_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tL3F1ZXN0aW9ucyJ9fQ==";
json.entries[3].url = "https://stackoverflow.com/tags";
json.entries[4] = {};
json.entries[4].ID = 512;
json.entries[4].cacheKey = 0;
json.entries[4].csp = "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";
json.entries[4].docIdentifier = 594;
json.entries[4].docshellUUID = "{b6c1e18a-d0f5-4ba2-996e-ddd9fc28322a}";
json.entries[4].hasUserInteraction = false;
json.entries[4].persist = true;
json.entries[4].principalToInherit_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tL3RhZ3MifX0=";
json.entries[4].referrerInfo = "BBoSnxDOS9qmDeAnom1e0AAAAAAAAAAAwAAAAAAAAEYBAAAAHmh0dHBzOi8vc3RhY2tvdmVyZmxvdy5jb20vdGFncwAAAAgBAQAAAB5odHRwczovL3N0YWNrb3ZlcmZsb3cuY29tL3RhZ3MBAQ==";
json.entries[4].resultPrincipalURI = null;
json.entries[4].title = "Users - Stack Overflow";
json.entries[4].triggeringPrincipal_base64 = "eyIxIjp7IjAiOiJodHRwczovL3N0YWNrb3ZlcmZsb3cuY29tL3RhZ3MifX0=";
json.entries[4].url = "https://stackoverflow.com/users";
json.hidden = false;
json.index = 3;
json.lastAccessed = 1651407567904;
json.requestedIndex = 0;
json.searchMode = null;
json.userContextId = 0

Answer 1

Just index entries by index minus one?
.entries[.index - 1]
Online demo

Answer 2

The last item in entries is not necessarily the tab that was opened. [.index - 1] only works if user_pref("browser.sessionstore.max_serialize_forward", 0); cf. https://wiki.mozilla.org/Firefox/session_restore#Browser_settings