process code with implicit return statement

Bookmarklet for instance implicitedly assumes a "completion value" without using `return`.
The `expression` option now supports such use cases.
Optimisations on IIFEs also enhanced.

fixes mishoo#354
fixes mishoo#543
fixes mishoo#625
fixes mishoo#628
fixes mishoo#640
closes mishoo#1293

Author: alexlamsl

README.md

@@ -426,6 +426,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
   such as `console.info` and/or retain side effects from function arguments
   after dropping the function call then use `pure_funcs` instead.
 
+- `expression` -- default `false`.  Pass `true` to preserve completion values
+  from terminal statements without `return`, e.g. in bookmarklets.
+
 - `keep_fargs` -- default `true`.  Prevents the
   compressor from discarding unused function arguments.  You need this
   for code which relies on `Function.length`.

lib/compress.js

@@ -80,6 +80,7 @@ function Compressor(options, false_by_default) {
         screw_ie8     : true,
         drop_console  : false,
         angular       : false,
+        expression    : false,
         warnings      : true,
         global_defs   : {},
         passes        : 1,
@@ -116,12 +117,18 @@ Compressor.prototype = new TreeTransformer;
 merge(Compressor.prototype, {
     option: function(key) { return this.options[key] },
     compress: function(node) {
+        if (this.option("expression")) {
+            node = node.process_expression(true);
+        }
         var passes = +this.options.passes || 1;
         for (var pass = 0; pass < passes && pass < 3; ++pass) {
             if (pass > 0 || this.option("reduce_vars"))
                 node.reset_opt_flags(this, true);
             node = node.transform(this);
         }
+        if (this.option("expression")) {
+            node = node.process_expression(false);
+        }
         return node;
     },
     warn: function(text, props) {
@@ -178,6 +185,42 @@ merge(Compressor.prototype, {
         return this.print_to_string() == node.print_to_string();
     });
 
+    AST_Node.DEFMETHOD("process_expression", function(insert) {
+        var self = this;
+        var tt = new TreeTransformer(function(node) {
+            if (insert && node instanceof AST_SimpleStatement) {
+                return make_node(AST_Return, node, {
+                    value: node.body
+                });
+            }
+            if (!insert && node instanceof AST_Return) {
+                return make_node(AST_SimpleStatement, node, {
+                    body: node.value || make_node(AST_Undefined, node)
+                });
+            }
+            if (node instanceof AST_Lambda && node !== self) {
+                return node;
+            }
+            if (node instanceof AST_Block) {
+                var index = node.body.length - 1;
+                if (index >= 0) {
+                    node.body[index] = node.body[index].transform(tt);
+                }
+            }
+            if (node instanceof AST_If) {
+                node.body = node.body.transform(tt);
+                if (node.alternative) {
+                    node.alternative = node.alternative.transform(tt);
+                }
+            }
+            if (node instanceof AST_With) {
+                node.body = node.body.transform(tt);
+            }
+            return node;
+        });
+        return self.transform(tt);
+    });
+
     AST_Node.DEFMETHOD("reset_opt_flags", function(compressor, rescan){
         var reduce_vars = rescan && compressor.option("reduce_vars");
         var safe_ids = [];
@@ -2030,7 +2073,14 @@ merge(Compressor.prototype, {
         def(AST_Constant, return_null);
         def(AST_This, return_null);
         def(AST_Call, function(compressor, first_in_statement){
-            if (!this.has_pure_annotation(compressor) && compressor.pure_funcs(this)) return this;
+            if (!this.has_pure_annotation(compressor) && compressor.pure_funcs(this)) {
+                if (this.expression instanceof AST_Function) {
+                    var node = this.clone();
+                    node.expression = node.expression.process_expression(false);
+                    return node;
+                }
+                return this;
+            }
             if (this.pure) {
                 compressor.warn("Dropping __PURE__ call [{file}:{line},{col}]", this.start);
                 this.pure.value = this.pure.value.replace(/[@#]__PURE__/g, ' ');
@@ -2522,12 +2572,13 @@ merge(Compressor.prototype, {
     });
 
     OPT(AST_Call, function(self, compressor){
+        var exp = self.expression;
         if (compressor.option("unused")
-            && self.expression instanceof AST_Function
-            && !self.expression.uses_arguments
-            && !self.expression.uses_eval
-            && self.args.length > self.expression.argnames.length) {
-            var end = self.expression.argnames.length;
+            && exp instanceof AST_Function
+            && !exp.uses_arguments
+            && !exp.uses_eval
+            && self.args.length > exp.argnames.length) {
+            var end = exp.argnames.length;
             for (var i = end, len = self.args.length; i < len; i++) {
                 var node = self.args[i].drop_side_effect_free(compressor);
                 if (node) {
@@ -2537,7 +2588,6 @@ merge(Compressor.prototype, {
             self.args.length = end;
         }
         if (compressor.option("unsafe")) {
-            var exp = self.expression;
             if (exp instanceof AST_SymbolRef && exp.undeclared()) {
                 switch (exp.name) {
                   case "Array":
@@ -2711,16 +2761,22 @@ merge(Compressor.prototype, {
                 return best_of(self, node);
             }
         }
-        if (compressor.option("side_effects")) {
-            if (self.expression instanceof AST_Function
-                && self.args.length == 0
-                && !AST_Block.prototype.has_side_effects.call(self.expression, compressor)) {
-                return make_node(AST_Undefined, self).transform(compressor);
+        if (exp instanceof AST_Function) {
+            if (exp.body[0] instanceof AST_Return
+                && exp.body[0].value.is_constant()) {
+                var args = self.args.concat(exp.body[0].value);
+                return AST_Seq.from_array(args).transform(compressor);
+            }
+            if (compressor.option("side_effects")) {
+                if (!AST_Block.prototype.has_side_effects.call(exp, compressor)) {
+                    var args = self.args.concat(make_node(AST_Undefined, self));
+                    return AST_Seq.from_array(args).transform(compressor);
+                }
             }
         }
         if (compressor.option("drop_console")) {
-            if (self.expression instanceof AST_PropAccess) {
-                var name = self.expression.expression;
+            if (exp instanceof AST_PropAccess) {
+                var name = exp.expression;
                 while (name.expression) {
                     name = name.expression;
                 }
@@ -2731,12 +2787,6 @@ merge(Compressor.prototype, {
                 }
             }
         }
-        if (self.args.length == 0
-            && self.expression instanceof AST_Function
-            && self.expression.body[0] instanceof AST_Return
-            && self.expression.body[0].value.is_constant()) {
-            return self.expression.body[0].value;
-        }
         if (compressor.option("negate_iife")
             && compressor.parent() instanceof AST_SimpleStatement
             && is_iife_call(self)) {

test/compress/drop-unused.js

@@ -632,7 +632,7 @@ iife: {
     }
     expect: {
         function f() {
-            ~function() {}(b);
+            b;
         }
     }
 }

test/compress/evaluate.js

@@ -640,9 +640,7 @@ call_args: {
     expect: {
         const a = 1;
         console.log(1);
-        +function(a) {
-            return 1;
-        }(1);
+        +(1, 1);
     }
 }
 
@@ -663,9 +661,7 @@ call_args_drop_param: {
     expect: {
         const a = 1;
         console.log(1);
-        +function() {
-            return 1;
-        }(b);
+        +(b, 1);
     }
 }
 

test/compress/functions.js

@@ -35,9 +35,9 @@ iifes_returning_constants_keep_fargs_true: {
         console.log("okay");
         console.log(123);
         console.log(void 0);
-        console.log(function(x,y,z){return 2}(1,2,3));
-        console.log(function(x,y){return 6}(2,3));
-        console.log(function(x, y){return 6}(2,3,a(),b()));
+        console.log(2);
+        console.log(6);
+        console.log((a(), b(), 6));
     }
 }
 
@@ -71,6 +71,6 @@ iifes_returning_constants_keep_fargs_false: {
         console.log(void 0);
         console.log(2);
         console.log(6);
-        console.log(function(){return 6}(a(),b()));
+        console.log((a(), b(), 6));
     }
 }