safely evaluate .length of arrays and strings

lib/compress/evaluate.js

@@ -352,16 +352,43 @@ const regexp_flags = new Set([
 def_eval(AST_PropAccess, function (compressor, depth) {
     let obj = this.expression._eval(compressor, depth + 1);
     if (obj === nullish || (this.optional && obj == null)) return nullish;
+
+    // `.length` of strings and arrays is always safe
+    if (this.property === "length") {
+        if (typeof obj === "string") {
+            return obj.length;
+        }
+
+        const is_spreadless_array = obj =>
+            obj instanceof AST_Array
+            && obj.elements.every(el => !(el instanceof AST_Expansion));
+
+        if (
+            is_spreadless_array(obj)
+            && obj.elements.every(el => !el.has_side_effects(compressor))
+        ) {
+            return obj.elements.length;
+        }
+
+        let fixed;
+        if (
+            obj instanceof AST_SymbolRef
+            && is_spreadless_array((fixed = obj.definition().fixed_value()))
+        ) {
+            return fixed.elements.length;
+        }
+    }
+
     if (compressor.option("unsafe")) {
         var key = this.property;
         if (key instanceof AST_Node) {
             key = key._eval(compressor, depth);
             if (key === this.property)
                 return this;
         }
+
         var exp = this.expression;
         if (is_undeclared_ref(exp)) {
-
             var aa;
             var first_arg = exp.name === "hasOwnProperty"
                 && key === "call"

test/compress/evaluate.js

@@ -805,6 +805,27 @@ unsafe_string_bad_index: {
     expect_stdout: true
 }
 
+safe_array_string_length: {
+    options = {
+        reduce_vars: true,
+        evaluate: true,
+    }
+    input: {
+        const array_ref = [1, leak("side effect")]
+        console.log(
+            "Hi!!".length,
+            [1, 2].length,
+            array_ref.length,
+            [id("side eff")]
+        );
+    }
+    expect: {
+        const array_ref = [1, leak("side effect")]
+        console.log(4, 2, 2, [id("side eff")]);
+    }
+    expect_stdout: "4 2 2 [ 'side eff' ]"
+}
+
 prototype_function: {
     options = {
         evaluate: true,