Switch an old script from Rhino to the V8 runtime

Originally written for bulldo.gs — republished here with the canonical link pointing home. I want to enable the V8 runtime on an existing Apps Script project so I can use modern JavaScript, but I am worried about breaking things that are already working. // appsscript.json — set runtimeVersion to enable V8 // Rollback: change V8 back to DEPRECATED_ES5 { " timeZone " : " America/New_York " , " dependencies " : {}, " exceptionLogging " : " STACKDRIVER " , " runtimeVersion " : " V8 " } // Code.gs — safe V8-compatible replacement for a common Rhino pattern // Rhino allowed: for each (var item in collection) {} // V8 requires standard for...of instead function listSheetNames () { var ss = SpreadsheetApp . getActiveSpreadsheet (); var names = []; var sheets = ss . getSheets (); for ( var i = 0 ; i < sheets . length ; i ++ ) { names . push ( sheets [ i ]. getName ()); } Logger . log ( names . join ( ' , ' )); } The one-line change and why it is a project-wide bomb Open the Apps Script editor, click Project Settings (the gear icon), and check "Show appsscript.json manifest file in editor." Then open that file and change "runtimeVersion": "DEPRECATED_ES5" to "runtimeVersion": "V8" . Save. That is the entire migration from a settings standpoint. What catches people off guard is the failure mode. V8 parses every .gs file in the project as a unit before running anything. One Rhino-only statement — a for each loop, a __iterator__ method, a Date.prototype.getYear call in an otherwise untouched utility file — causes a syntax or runtime error that prevents the whole script from initializing. Not just the file that contains the bad line. Every function, every trigger, the entire project goes dark. The first time I hit this it took me twenty minutes to figure out why a completely unrelated trigger had stopped firing. The error message pointed at the Rhino syntax in a helper file I had not touched in two years. V8 does not isolate the damage; it fails at parse time, before any executi