From 38a46c86d7163002ad079eb9dd690e18b5c9da8f Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Fri, 18 Sep 2020 14:35:29 +0100
Subject: [PATCH] enhance `side_effects` (#4124)

- add documentation for `merge_vars`
---
 README.md                     |  2 ++
 lib/compress.js               |  3 ++-
 test/compress/side_effects.js | 25 +++++++++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 964faf46..6c549768 100644
--- a/README.md
+++ b/README.md
@@ -688,6 +688,8 @@ to be `false` and all symbol names will be omitted.
 - `loops` (default: `true`) -- optimizations for `do`, `while` and `for` loops
   when we can statically determine the condition.
 
+- `merge_vars` (default: `true`) -- combine and reuse variables.
+
 - `negate_iife` (default: `true`) -- negate "Immediately-Called Function Expressions"
   where the return value is discarded, to avoid the parens that the
   code generator would insert.
diff --git a/lib/compress.js b/lib/compress.js
index 61d6079c..cfd3e2b2 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3892,7 +3892,8 @@ merge(Compressor.prototype, {
             if (is_undeclared_ref(expr) && global_pure_fns[expr.name]) return true;
             if (expr instanceof AST_Dot && is_undeclared_ref(expr.expression)) {
                 var static_fn = static_fns[expr.expression.name];
-                return static_fn && static_fn[expr.property];
+                return static_fn && (static_fn[expr.property]
+                    || expr.expression.name == "Math" && expr.property == "random");
             }
         }
         return this.pure || !compressor.pure_funcs(this);
diff --git a/test/compress/side_effects.js b/test/compress/side_effects.js
index 244e3881..0746a56d 100644
--- a/test/compress/side_effects.js
+++ b/test/compress/side_effects.js
@@ -245,6 +245,31 @@ unsafe_builtin_2: {
     expect_stdout: "object PASS PASS"
 }
 
+unsafe_builtin_3: {
+    options = {
+        conditionals: true,
+        side_effects: true,
+        toplevel: true,
+        unsafe: true,
+    }
+    input: {
+        var o = {};
+        if (42 < Math.random())
+            o.p = "FAIL";
+        else
+            o.p = "PASS";
+        for (var k in o)
+            console.log(k, o[k]);
+    }
+    expect: {
+        var o = {};
+        o.p = 42 < Math.random() ? "FAIL" : "PASS";
+        for (var k in o)
+            console.log(k, o[k]);
+    }
+    expect_stdout: "p PASS"
+}
+
 unsafe_string_replace: {
     options = {
         side_effects: true,
-- 
2.34.1