Why Copy-Pasting Rhino-Era Code is Dangerous: 5 “Old GAS Styles” That Break in V8 [For Beginners]

When researching Google Apps Script (GAS), you might copy-paste code from blogs, Stack Overflow, or old documentation.

However, you need to be careful.

That code might be written in the Rhino era style. And Rhino support will end on January 31, 2026. In V8, there are writing styles that "will break if left as is".

In this article, I will explain 5 Rhino-era writing styles that are likely to break when switching to V8, including "why it's bad" and "how to fix it".

Premise: Why is "Rhino-era code" dangerous?

In other words,

Code that "happened to work" in Rhino will "normally result in an error" in V8.

Bad Example 1: Using for each … in

❌ Common code in the Rhino era

for each (var row in values) {
  Logger.log(row);
}

Why is it bad?

for each … in is not a standard JavaScript specification. It is a Rhino-specific extension.

👉 In V8, it is completely unsupported, so it causes an error.

✅ Correct writing in V8

for (var row of values) {
  console.log(row);
}

values.forEach(function(row) {
  console.log(row);
});

Bad Example 2: Scope-dependent code assuming var

❌ Code common in the Rhino era

for (var i = 0; i < 5; i++) {
  setTimeout(function() {
    Logger.log(i);
  }, 1000);
}

What happens?

In Rhino, there were cases where it worked "somewhat as expected", but in V8, the behavior becomes clear, such as all outputting the same value (5).

Why is it bad?

var has function scope. In V8, this behavior is evaluated more strictly, and bugs become apparent.

✅ Correct answer in V8

for (let i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i);
  }, 1000);
}

👉 In V8, using let / const is the standard.

Bad Example 3: Using reserved words for variable names

❌ Example that works in Rhino

var class = 'test';
var default = 1;

Why is it bad?

class and default are JavaScript reserved words.

• Rhino: Silently allowed it
• V8: Immediate error

✅ Fix example in V8

var className = 'test';
var defaultValue = 1;

👉 Do not use reserved words for variable names. Older articles often contain this landmine.

Bad Example 4: Using Date.getYear()

❌ Code common in the Rhino era

var year = new Date().getYear();

What is bad?

getYear() is a method with quite quirky specifications.

• 2025 → returns 125
• Often used without noticing in Rhino
• In V8, it works "according to spec", so the year becomes strange.

✅ Correct writing

var year = new Date().getFullYear();

👉 This is a representative example where actual harm is likely to occur in V8 migration.

Bad Example 5: Implicit global variables

❌ Code that passed in Rhino

function test() {
  x = 10;
}

Why is it bad?

Variables without var / let / const are:

• Rhino: Treated as implicit globals
• V8: Error or unexpected behavior

✅ Correct answer in V8

function test() {
  let x = 10;
}

👉 In V8, "undeclared variables" are a source of accidents.

Why "Copy-Paste Code" is the Most Dangerous

1. The person who wrote it doesn't understand the behavior
2. It's hard to tell if it's written assuming Rhino
3. You assume "it's OK because it's working"

ZIDOOKA! Summary

【Summary Article】 Here is the complete guide summarizing all information about "Rhino Deprecation / V8 Migration" including this article. 👉 Complete Guide to GAS Rhino Deprecation & V8 Migration