{?exists} and {^exists} resolve Promises and check if the result exis…

…ts (linkedin#753)

Closes linkedin#752

lib/dust.js

@@ -725,6 +725,11 @@
     return this;
   };
 
+  /**
+   * Inserts a new chunk that can be used to asynchronously render or write to it
+   * @param callback {Function} The function that will be called with the new chunk
+   * @returns {Chunk} A copy of this chunk instance in order to further chain function calls on the chunk
+   */
   Chunk.prototype.map = function(callback) {
     var cursor = new Chunk(this.root, this.next, this.taps),
         branch = new Chunk(this.root, cursor, this.taps);
@@ -740,6 +745,35 @@
     return cursor;
   };
 
+  /**
+   * Like Chunk#map but additionally resolves a thenable.  If the thenable succeeds the callback is invoked with
+   * a new chunk that can be used to asynchronously render or write to it, otherwise if the thenable is rejected 
+   * then the error body is rendered if available, an error is logged, and the callback is never invoked.
+   * @param {Chunk} The current chunk to insert a new chunk
+   * @param thenable {Thenable} the target thenable to await
+   * @param context {Context} context to use to render the deferred chunk
+   * @param bodies {Object} may optionally contain an "error" for when the thenable is rejected
+   * @param callback {Function} The function that will be called with the new chunk
+   * @returns {Chunk} A copy of this chunk instance in order to further chain function calls on the chunk
+   */
+  function mapThenable(chunk, thenable, context, bodies, callback) {
+    return chunk.map(function(asyncChunk) {
+      thenable.then(function(data) {
+        try {
+          callback(asyncChunk, data);
+        } catch (err) {
+          // handle errors the same way Chunk#map would.  This logic is only here since the thenable defers 
+          // logic such that the try / catch in Chunk#map would not capture it. 
+          dust.log(err, ERROR);
+          asyncChunk.setError(err);
+        }
+      }, function(err) {
+        dust.log('Unhandled promise rejection in `' + context.getTemplateName() + '`', INFO);
+        asyncChunk.renderError(err, context, bodies).end();
+      });
+    });
+  }
+
   Chunk.prototype.tap = function(tap) {
     var taps = this.taps;
 
@@ -861,6 +895,12 @@
     var body = bodies.block,
         skip = bodies['else'];
 
+    if (dust.isThenable(elem)) {
+      return mapThenable(this, elem, context, bodies, function(chunk, data) {
+        chunk.exists(data, context, bodies).end();
+      });
+    }
+
     if (!dust.isEmpty(elem)) {
       if (body) {
         return body(this, context);
@@ -876,6 +916,12 @@
     var body = bodies.block,
         skip = bodies['else'];
 
+    if (dust.isThenable(elem)) {
+      return mapThenable(this, elem, context, bodies, function(chunk, data) {
+        chunk.notexists(data, context, bodies).end();
+      });
+    }
+
     if (dust.isEmpty(elem)) {
       if (body) {
         return body(this, context);
@@ -970,27 +1016,31 @@
    * @return {Chunk}
    */
   Chunk.prototype.await = function(thenable, context, bodies, auto, filters) {
-    return this.map(function(chunk) {
-      thenable.then(function(data) {
-        if (bodies) {
-          chunk = chunk.section(data, context, bodies);
-        } else {
-          // Actually a reference. Self-closing sections don't render
-          chunk = chunk.reference(data, context, auto, filters);
-        }
-        chunk.end();
-      }, function(err) {
-        var errorBody = bodies && bodies.error;
-        if(errorBody) {
-          chunk.render(errorBody, context.push(err)).end();
-        } else {
-          dust.log('Unhandled promise rejection in `' + context.getTemplateName() + '`', INFO);
-          chunk.end();
-        }
-      });
+    return mapThenable(this, thenable, context, bodies, function(chunk, data) {
+      if (bodies) {
+        chunk.section(data, context, bodies).end();
+      } else {
+        // Actually a reference. Self-closing sections don't render
+        chunk.reference(data, context, auto, filters).end();
+      }
     });
   };
 
+  /**
+   * Render an error body if available
+   * @param err {Error} error that occurred
+   * @param context {Context} context to use to render the error
+   * @param bodies {Object} may optionally contain an "error" which will be rendered
+   * @return {Chunk}
+   */
+  Chunk.prototype.renderError = function(err, context, bodies) {
+    var errorBody = bodies && bodies.error;
+    if (errorBody) {
+      return this.render(errorBody, context.push(err));
+    }
+    return this;
+  };
+
   /**
    * Reserve a chunk to be evaluated with the contents of a streamable.
    * Currently an error event will bomb out the stream. Once an error
@@ -1002,8 +1052,7 @@
    * @return {Chunk}
    */
   Chunk.prototype.stream = function(stream, context, bodies, auto, filters) {
-    var body = bodies && bodies.block,
-        errorBody = bodies && bodies.error;
+    var body = bodies && bodies.block;
     return this.map(function(chunk) {
       var ended = false;
       stream
@@ -1025,11 +1074,8 @@
           if(ended) {
             return;
           }
-          if(errorBody) {
-            chunk.render(errorBody, context.push(err));
-          } else {
-            dust.log('Unhandled stream error in `' + context.getTemplateName() + '`', INFO);
-          }
+          chunk.renderError(err, context, bodies);
+          dust.log('Unhandled stream error in `' + context.getTemplateName() + '`', INFO);
           if(!ended) {
             ended = true;
             chunk.end();

test/templates.spec.js

@@ -93,7 +93,7 @@ function render(test, dust) {
         expect(messageInLog(dust.logQueue, test.log)).toEqual(true);
       }
       if (typeof test.expected !== 'undefined') {
-        expect(test.expected).toEqual(output);
+        expect(output).toEqual(test.expected);
       }
       done();
     };
@@ -124,7 +124,7 @@ function stream(test, dust) {
         expect(messageInLog(dust.logQueue, test.log)).toEqual(true);
       }
       if (typeof test.expected !== 'undefined') {
-        expect(test.expected).toEqual(result.output);
+        expect(result.output).toEqual(test.expected);
       }
       done();
     };
@@ -196,7 +196,7 @@ function pipe(test, dust) {
         expect(messageInLog(dust.logQueue, test.log)).toEqual(true);
       }
       if (typeof test.expected !== 'undefined') {
-        expect(test.expected).toEqual(result.data);
+        expect(result.data).toEqual(test.expected);
       }
       if(calls === 2) {
         done();

test/templates/all.js

@@ -262,6 +262,18 @@ return [
         message: "should setup base template for next test. hi should not be part of base block name"
 
       },
+      {
+        name:     "{?exists} supports promises and uses correct context",
+        source:   "{#a}{?b}{test}{/b}{/a}",
+        context:  {
+          a: {
+            b: FalsePromise(null, { test: "BAD" }),
+            test: "GOOD"
+          }
+        },
+        expected: "GOOD",
+        message:  "{?exists} supports promises and uses correct context",      
+      },
       {
         name:     "issue322 use base template picks up prefix chunk data",
         source:   '{>issue322 name="abc"/}' +
@@ -584,6 +596,13 @@ return [
         expected: "false",
         message:  "empty array is treated as empty in exists"
       },
+      {
+        name:     "empty array resolved from a Promise is treated as empty in exists",
+        source:   "{?emptyArrayFromPromise}true{:else}false{/emptyArrayFromPromise}",
+        context:   {"emptyArrayFromPromise": FalsePromise(null, [])},
+        expected: "false",
+        message:  "empty array resolved from a Promise is treated as empty in exists"
+      },
       {
         name:     "empty {} is treated as non empty in exists",
         source:   "{?object}true{:else}false{/object}",