Description

When property_write_side_effects is false, the minifier removes unused property writes like obj.a = 1 if all references to obj are member write targets. However, if __proto__ is assigned after the property write (e.g., via a hoisted function), the setter installed by __proto__ isn't detected, and the property write is incorrectly removed.

Reproduction

const obj = {}
f()
obj.a = 1

function f() {
  obj.__proto__ = { set a(v) { console.log("hello") } }
}

Currently obj.a = 1 may be removed, but at runtime f() installs a setter on obj via __proto__, so obj.a = 1 triggers console.log("hello") and must be preserved.

Root Cause

The current implementation tracks __proto__ writes as it encounters them during traversal. Since AST traversal is top-down, a __proto__ write inside a hoisted function that appears later in the source isn't recorded when the earlier obj.a = 1 is visited.

Possible Fix

Pre-scan all member write references to detect __proto__ writes before the removal pass, so that the tracking is complete regardless of source order.

Related

• PR feat(minifier): support property_write_side_effects option to drop unused property assignments #20773 — original property_write_side_effects implementation
• Review comment