From a6e0ec83716685882af4328e5f622523d3df237f Mon Sep 17 00:00:00 2001
From: Andrew Imm <andrewi@fb.com>
Date: Mon, 31 Aug 2015 17:38:38 -0700
Subject: [PATCH] Allow more certainty when evaluating Logical Expressions

---
 .../babel/src/traversal/path/evaluation.js    |  20 ++-
 packages/babel/test/evaluation.js             | 164 ++++++++++++++++++
 2 files changed, 182 insertions(+), 2 deletions(-)
 create mode 100644 packages/babel/test/evaluation.js

diff --git a/packages/babel/src/traversal/path/evaluation.js b/packages/babel/src/traversal/path/evaluation.js
index 36c6f159f6..50ab6ce623 100644
--- a/packages/babel/src/traversal/path/evaluation.js
+++ b/packages/babel/src/traversal/path/evaluation.js
@@ -134,12 +134,28 @@ export function evaluate(): { confident: boolean; value: any } {
     }
 
     if (path.isLogicalExpression()) {
+      // If we are confident that one side of an && is false, or one side of
+      // an || is true, we can be confident about the entire expression
+      let wasConfident = confident;
       let left = evaluate(path.get("left"));
+      let leftConfident = confident;
+      confident = wasConfident;
       let right = evaluate(path.get("right"));
+      let rightConfident = confident;
+      let uncertain = leftConfident !== rightConfident;
+      confident = leftConfident && rightConfident;
 
       switch (node.operator) {
-        case "||": return left || right;
-        case "&&": return left && right;
+        case "||":
+          if ((left || right) && uncertain) {
+            confident = true;
+          }
+          return left || right;
+        case "&&":
+          if ((!left && leftConfident) || (!right && rightConfident)) {
+            confident = true;
+          }
+          return left && right;
       }
     }
 
diff --git a/packages/babel/test/evaluation.js b/packages/babel/test/evaluation.js
new file mode 100644
index 0000000000..632691bb0d
--- /dev/null
+++ b/packages/babel/test/evaluation.js
@@ -0,0 +1,164 @@
+var evaluation = require("../lib/traversal/path/evaluation");
+var traverse   = require('../lib/traversal');
+var parse      = require("../lib/helpers/parse");
+var assert     = require("assert");
+
+suite("evaluation", function () {
+  test("binary expression", function () {
+    traverse(parse("5 + 5"), {
+      enter: function (node) {
+        if (this.isBinaryExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: 10 });
+        }
+      }
+    });
+
+    traverse(parse("'str' === 'str'"), {
+      enter: function(node) {
+        if (this.isBinaryExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'four' === 4"), {
+      enter: function(node) {
+        if (this.isBinaryExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+  });
+
+  test("logical expression", function () {
+    traverse(parse("'abc' === 'abc' && 1 === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'abc' && 1 === 10"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' && 1 === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' && 1 === 10"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'abc' || 1 === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'abc' || 1 === 10"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' || 1 === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' || 1 === 10"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+  });
+
+  test("logical expression without certainty", function () {
+    traverse(parse("'abc' === 'abc' || config.flag === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("obj.a === 'abc' || config.flag === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: false, value: undefined });
+        }
+      }
+    });
+
+    traverse(parse("'abc' !== 'abc' && config.flag === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: false });
+        }
+      }
+    });
+
+    traverse(parse("obj.a === 'abc' && 1 === 1"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: false, value: undefined });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'abc' && (1 === 1 || config.flag)"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' || (1 === 1 && config.flag)"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: false, value: undefined });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'xyz' || (1 === 1 && 'four' === 'four')"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+
+    traverse(parse("'abc' === 'abc' && (1 === 1 && 'four' === 'four')"), {
+      enter: function(node) {
+        if (this.isLogicalExpression()) {
+          assert.deepEqual(this.evaluate(), { confident: true, value: true });
+        }
+      }
+    });
+  });
+});