improve usability of global_defs in minify() (#1987)

Use `@key` to `parse()` string value as `AST_Node`.

fixes #1986

README.md

@@ -786,7 +786,7 @@ You can also use conditional compilation via the programmatic API. With the diff
 property name is `global_defs` and is a compressor property:
 
 ```javascript
-var result = uglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
+var result = UglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
     compress: {
         dead_code: true,
         global_defs: {
@@ -796,6 +796,32 @@ var result = uglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
 });
 ```
 
+To replace an identifier with an arbitrary non-constant expression it is
+necessary to prefix the `global_defs` key with `"@"` to instruct UglifyJS
+to parse the value as an expression:
+```javascript
+UglifyJS.minify("alert('hello');", {
+    compress: {
+        global_defs: {
+            "@alert": "console.log"
+        }
+    }
+}).code;
+// returns: 'console.log("hello");'
+```
+
+Otherwise it would be replaced as string literal:
+```javascript
+UglifyJS.minify("alert('hello');", {
+    compress: {
+        global_defs: {
+            "alert": "console.log"
+        }
+    }
+}).code;
+// returns: '"console.log"("hello");'
+```
+
 ### Using native Uglify AST with `minify()`
 ```javascript
 // example: parse only, produce native Uglify AST

lib/compress.js

@@ -87,6 +87,17 @@ function Compressor(options, false_by_default) {
         unused        : !false_by_default,
         warnings      : false,
     }, true);
+    var global_defs = this.options["global_defs"];
+    if (typeof global_defs == "object") for (var key in global_defs) {
+        if (/^@/.test(key) && HOP(global_defs, key)) {
+            var ast = parse(global_defs[key]);
+            if (ast.body.length == 1 && ast.body[0] instanceof AST_SimpleStatement) {
+                global_defs[key.slice(1)] = ast.body[0].body;
+            } else throw new Error(string_template("Can't handle expression: {value}", {
+                value: global_defs[key]
+            }));
+        }
+    }
     var pure_funcs = this.options["pure_funcs"];
     if (typeof pure_funcs == "function") {
         this.pure_funcs = pure_funcs;

test/compress/global_defs.js

@@ -160,3 +160,17 @@ issue_1801: {
         console.log(!0);
     }
 }
+
+issue_1986: {
+    options = {
+        global_defs: {
+            "@alert": "console.log",
+        },
+    }
+    input: {
+        alert(42);
+    }
+    expect: {
+        console.log(42);
+    }
+}

test/mocha/minify.js

@@ -181,4 +181,19 @@ describe("minify", function() {
             assert.strictEqual(err.col, 12);
         });
     });
+
+    describe("global_defs", function() {
+        it("should throw for non-trivial expressions", function() {
+            var result = Uglify.minify("alert(42);", {
+                compress: {
+                    global_defs: {
+                        "@alert": "debugger"
+                    }
+                }
+            });
+            var err = result.error;
+            assert.ok(err instanceof Error);
+            assert.strictEqual(err.stack.split(/\n/)[0], "Error: Can't handle expression: debugger");
+        });
+    });
 });