feat($parse): added `constant` and `literal` properties

* `literal` is set to true if the expression's top-level is a JavaScript
  literal (number, string, boolean, null/undefined, array, object), even
  if it contains non-literals inside.

* `constant` is set to true if the expression is known to be made
  entirely of constant values, i.e., evaluating it will always yield the
  same result.

A consequence is that a JSON expression is guaranteed to be both literal
and constant.

2 files changed, 108 insertions, 11 deletions

diff --git a/src/ng/parse.js b/src/ng/parse.js
index 60181f96..4651fc10 100644
--- a/src/ng/parse.js
+++ b/src/ng/parse.js
@@ -303,6 +303,8 @@ function parser(text, json, $filter, csp){
   if (tokens.length !== 0) {
     throwError("is an unexpected token", tokens[0]);
   }
+  value.literal = !!value.literal;
+  value.constant = !!value.constant;
   return value;
 
   ///////////////////////////////////
@@ -350,15 +352,19 @@ function parser(text, json, $filter, csp){
   }
 
   function unaryFn(fn, right) {
-    return function(self, locals) {
+    return extend(function(self, locals) {
       return fn(self, locals, right);
-    };
+    }, {
+      constant:right.constant
+    });
   }
 
   function binaryFn(left, fn, right) {
-    return function(self, locals) {
+    return extend(function(self, locals) {
       return fn(self, locals, left, right);
-    };
+    }, {
+      constant:left.constant && right.constant
+    });
   }
 
   function statements() {
@@ -526,6 +532,9 @@ function parser(text, json, $filter, csp){
       if (!primary) {
         throwError("not a primary expression", token);
       }
+      if (token.json) {
+        primary.constant = primary.literal = true;
+      }
     }
 
     var next, context;
@@ -614,23 +623,32 @@ function parser(text, json, $filter, csp){
   // This is used with json array declaration
   function arrayDeclaration () {
     var elementFns = [];
+    var allConstant = true;
     if (peekToken().text != ']') {
       do {
-        elementFns.push(expression());
+        var elementFn = expression();
+        elementFns.push(elementFn);
+        if (!elementFn.constant) {
+          allConstant = false;
+        }
       } while (expect(','));
     }
     consume(']');
-    return function(self, locals){
+    return extend(function(self, locals){
       var array = [];
       for ( var i = 0; i < elementFns.length; i++) {
         array.push(elementFns[i](self, locals));
       }
       return array;
-    };
+    }, {
+      literal:true,
+      constant:allConstant
+    });
   }
 
   function object () {
     var keyValues = [];
+    var allConstant = true;
     if (peekToken().text != '}') {
       do {
         var token = expect(),
@@ -638,10 +656,13 @@ function parser(text, json, $filter, csp){
         consume(":");
         var value = expression();
         keyValues.push({key:key, value:value});
+        if (!value.constant) {
+          allConstant = false;
+        }
       } while (expect(','));
     }
     consume('}');
-    return function(self, locals){
+    return extend(function(self, locals){
       var object = {};
       for ( var i = 0; i < keyValues.length; i++) {
         var keyValue = keyValues[i];
@@ -649,7 +670,10 @@ function parser(text, json, $filter, csp){
         object[keyValue.key] = value;
       }
       return object;
-    };
+    }, {
+      literal:true,
+      constant:allConstant
+    });
   }
 }
 
@@ -853,8 +877,13 @@ function getterFn(path, csp) {
  *    * `locals` – `{object=}` – local variables context object, useful for overriding values in
  *      `context`.
  *
- *    The return function also has an `assign` property, if the expression is assignable, which
- *    allows one to set values to expressions.
+ *    The returned function also has the following properties:
+ *      * `literal` – `{boolean}` – whether the expression's top-level node is a JavaScript
+ *        literal.
+ *      * `constant` – `{boolean}` – whether the expression is made entirely of JavaScript
+ *        constant literals.
+ *      * `assign` – `{?function(context, value)}` – if the expression is assignable, this will be
+ *        set to a function to change its value on the given context.
  *
  */
 function $ParseProvider() {
diff --git a/test/ng/parseSpec.js b/test/ng/parseSpec.js
index a6f43282..020a1d6f 100644
--- a/test/ng/parseSpec.js
+++ b/test/ng/parseSpec.js
@@ -671,6 +671,74 @@ describe('parser', function() {
           expect($parse('a.b')({a: {b: 0}}, {a: null})).toEqual(undefined);
         }));
       });
+
+      describe('literal', function() {
+        it('should mark scalar value expressions as literal', inject(function($parse) {
+          expect($parse('0').literal).toBe(true);
+          expect($parse('"hello"').literal).toBe(true);
+          expect($parse('true').literal).toBe(true);
+          expect($parse('false').literal).toBe(true);
+          expect($parse('null').literal).toBe(true);
+          expect($parse('undefined').literal).toBe(true);
+        }));
+
+        it('should mark array expressions as literal', inject(function($parse) {
+          expect($parse('[]').literal).toBe(true);
+          expect($parse('[1, 2, 3]').literal).toBe(true);
+          expect($parse('[1, identifier]').literal).toBe(true);
+        }));
+
+        it('should mark object expressions as literal', inject(function($parse) {
+          expect($parse('{}').literal).toBe(true);
+          expect($parse('{x: 1}').literal).toBe(true);
+          expect($parse('{foo: bar}').literal).toBe(true);
+        }));
+
+        it('should not mark function calls or operator expressions as literal', inject(function($parse) {
+          expect($parse('1 + 1').literal).toBe(false);
+          expect($parse('call()').literal).toBe(false);
+          expect($parse('[].length').literal).toBe(false);
+        }));
+      });
+
+      describe('constant', function() {
+        it('should mark scalar value expressions as constant', inject(function($parse) {
+          expect($parse('12.3').constant).toBe(true);
+          expect($parse('"string"').constant).toBe(true);
+          expect($parse('true').constant).toBe(true);
+          expect($parse('false').constant).toBe(true);
+          expect($parse('null').constant).toBe(true);
+          expect($parse('undefined').constant).toBe(true);
+        }));
+
+        it('should mark arrays as constant if they only contain constant elements', inject(function($parse) {
+          expect($parse('[]').constant).toBe(true);
+          expect($parse('[1, 2, 3]').constant).toBe(true);
+          expect($parse('["string", null]').constant).toBe(true);
+          expect($parse('[[]]').constant).toBe(true);
+          expect($parse('[1, [2, 3], {4: 5}]').constant).toBe(true);
+        }));
+
+        it('should not mark arrays as constant if they contain any non-constant elements', inject(function($parse) {
+          expect($parse('[foo]').constant).toBe(false);
+          expect($parse('[x + 1]').constant).toBe(false);
+          expect($parse('[bar[0]]').constant).toBe(false);
+        }));
+
+        it('should mark complex expressions involving constant values as constant', inject(function($parse) {
+          expect($parse('!true').constant).toBe(true);
+          expect($parse('1 - 1').constant).toBe(true);
+          expect($parse('"foo" + "bar"').constant).toBe(true);
+          expect($parse('5 != null').constant).toBe(true);
+          expect($parse('{standard: 4/3, wide: 16/9}').constant).toBe(true);
+        }));
+
+        it('should not mark any expression involving variables or function calls as constant', inject(function($parse) {
+          expect($parse('true.toString()').constant).toBe(false);
+          expect($parse('foo(1, 2, 3)').constant).toBe(false);
+          expect($parse('"name" + id').constant).toBe(false);
+        }));
+      });
     });
   });
 });