Implement JEP-10: Slice Projections

Same implementation as python, this implements
JEP-10 from jmespath/jmespath.site#3.

Author: jamesls

jmespath.js

@@ -530,7 +530,8 @@
 
       nudLbracket: function() {
           if (this.lookahead(0) === "Number" || this.lookahead(0) === "Colon") {
-              return this.parseIndexExpression();
+              var right = this.parseIndexExpression();
+              return this.projectIfSlice({type: "Identity"}, right);
           } else if (this.lookahead(0) === "Star" &&
                      this.lookahead(1) === "Rbracket") {
               this.advance();
@@ -556,6 +557,18 @@
           }
       },
 
+      projectIfSlice: function(left, right) {
+          var indexExpr = {type: "IndexExpression", children: [left, right]};
+          if (right.type === "Slice") {
+              return {
+                  type: "Projection",
+                  children: [indexExpr, this.parseProjectionRHS(this.bindingPower.Star)]
+              };
+          } else {
+              return indexExpr;
+          }
+      },
+
       parseSliceExpression: function() {
           // [start:end:step] where each part is optional, as well as the last
           // colon.
@@ -653,7 +666,7 @@
           var right;
           if (token.type === "Number" || token.type === "Colon") {
               right = this.parseIndexExpression();
-              return {type: "IndexExpression", children: [left, right]};
+              return this.projectIfSlice(left, right);
           } else {
               this.match("Star");
               this.match("Rbracket");

test/compliance/slice.json

@@ -127,4 +127,52 @@
       "error": "syntax"
     }
   ]
+}, {
+  "given": {
+    "foo": [{"a": 1}, {"a": 2}, {"a": 3}],
+    "bar": [{"a": {"b": 1}}, {"a": {"b": 2}},
+	    {"a": {"b": 3}}]
+  },
+  "cases": [
+    {
+      "expression": "foo[:2].a",
+      "result": [1, 2]
+    },
+    {
+      "expression": "foo[:2].b",
+      "result": []
+    },
+    {
+      "expression": "foo[:2].a.b",
+      "result": []
+    },
+    {
+      "expression": "bar[::-1].a.b",
+      "result": [3, 2, 1]
+    },
+    {
+      "expression": "bar[:2].a.b",
+      "result": [1, 2]
+    }
+  ]
+}, {
+  "given": [{"a": 1}, {"a": 2}, {"a": 3}],
+  "cases": [
+    {
+      "expression": "[:]",
+      "result": [{"a": 1}, {"a": 2}, {"a": 3}]
+    },
+    {
+      "expression": "[:2].a",
+      "result": [1, 2]
+    },
+    {
+      "expression": "[::-1].a",
+      "result": [3, 2, 1]
+    },
+    {
+      "expression": "[:2].b",
+      "result": []
+    }
+  ]
 }]