diff --git a/lib/compress.js b/lib/compress.js
index b515c4259c..80e967ed1f 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1275,7 +1275,14 @@ Compressor.prototype.compress = function(node) {
             descend();
             var node = this;
             var expr = node.expression;
-            if (!node.optional && expr instanceof AST_SymbolRef) access(tw, expr.definition());
+            if (!node.optional) {
+                while (expr instanceof AST_Assign && expr.operator == "=") {
+                    var lhs = expr.left;
+                    if (lhs instanceof AST_SymbolRef) access(tw, lhs.definition());
+                    expr = expr.right;
+                }
+                if (expr instanceof AST_SymbolRef) access(tw, expr.definition());
+            }
             return true;
         });
         def(AST_For, function(tw, descend, compressor) {
@@ -1385,6 +1392,11 @@ Compressor.prototype.compress = function(node) {
                 pop(tw);
             } else {
                 descend();
+                while (expr instanceof AST_Assign && expr.operator == "=") {
+                    var lhs = expr.left;
+                    if (lhs instanceof AST_SymbolRef) access(tw, lhs.definition());
+                    expr = expr.right;
+                }
                 if (expr instanceof AST_SymbolRef) access(tw, expr.definition());
             }
             return true;
@@ -8056,6 +8068,7 @@ Compressor.prototype.compress = function(node) {
                 if (node.write_only === "p" && node.right.may_throw_on_access(compressor, true)) return;
                 var assign = props.assign;
                 if (assign) {
+                    initializations.add(node_def.id, assign.left);
                     assign.write_only = true;
                     assign.walk(tw);
                 }
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index fffa69419f..06c0f46ae4 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -3814,3 +3814,59 @@ issue_5533_drop_fargs: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5908_1: {
+    options = {
+        collapse_vars: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = function(b) {
+            function f() {}
+            b = f.prototype;
+            b.p = 42;
+            b.q = "PASS";
+            return f;
+        }();
+        console.log(a.prototype.q);
+    }
+    expect: {
+        var a = function(b) {
+            function f() {}
+            (b = f.prototype).p = 42;
+            b.q = "PASS";
+            return f;
+        }();
+        console.log(a.prototype.q);
+    }
+    expect_stdout: "PASS"
+}
+
+issue_5908_2: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = function(b) {
+            function f() {}
+            (b = f.prototype).p = 42;
+            b.q = "PASS";
+            return f;
+        }();
+        console.log(a.prototype.q);
+    }
+    expect: {
+        var a = function(b) {
+            function f() {}
+            (b = f.prototype).p = 42;
+            b.q = "PASS";
+            return f;
+        }();
+        console.log(a.prototype.q);
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/functions.js b/test/compress/functions.js
index 42a2acf995..d900d32809 100644
--- a/test/compress/functions.js
+++ b/test/compress/functions.js
@@ -7139,7 +7139,7 @@ reduce_cross_reference_2_toplevel: {
 reduce_cross_reference_3: {
     options = {
         collapse_vars: true,
-        passes: 3,
+        passes: 4,
         pure_getters: "strict",
         reduce_vars: true,
         sequences: true,