diff --git a/lib/compress.js b/lib/compress.js
index 530e7c2fee5..3423314329b 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2104,6 +2104,34 @@ merge(Compressor.prototype, {
             }
             break;
         }
+        if (compressor.option("conditionals")) {
+            if (self.operator == "&&") {
+                var ll = self.left.evaluate(compressor);
+                var rr = self.right.evaluate(compressor);
+                if (ll.length > 1) {
+                    if (ll[1]) {
+                        compressor.warn("Condition left of && always true [{file}:{line},{col}]", self.start);
+                        return rr[0];
+                    } else {
+                        compressor.warn("Condition left of && always false [{file}:{line},{col}]", self.start);
+                        return ll[0];
+                    }
+                }
+            }
+            if (self.operator == "||") {
+                var ll = self.left.evaluate(compressor);
+                var rr = self.right.evaluate(compressor);
+                if (ll.length > 1) {
+                    if (ll[1]) {
+                        compressor.warn("Condition left of || always true [{file}:{line},{col}]", self.start);
+                        return ll[0];
+                    } else {
+                        compressor.warn("Condition left of || always false [{file}:{line},{col}]", self.start);
+                        return rr[0];
+                    }
+                }
+            }
+        }
         if (compressor.option("booleans") && compressor.in_boolean_context()) switch (self.operator) {
           case "&&":
             var ll = self.left.evaluate(compressor);
diff --git a/test/compress/conditionals.js b/test/compress/conditionals.js
index 1b961812717..78f087dbd25 100644
--- a/test/compress/conditionals.js
+++ b/test/compress/conditionals.js
@@ -384,3 +384,164 @@ cond_8: {
         a = condition ? 1 : 0;
     }
 }
+
+conditional_and: {
+    options = {
+        conditionals: true,
+        evaluate    : true
+    };
+    input: {
+        var a;
+        // compress these
+
+        a = true     && condition;
+        a = 1        && console.log("a");
+        a = 2 * 3    && 2 * condition;
+        a = 5 == 5   && condition + 3;
+        a = "string" && 4 - condition;
+        a = 5 + ""   && condition / 5;
+        a = -4.5     && 6 << condition;
+        a = 6        && 7;
+
+        a = false     && condition;
+        a = NaN       && console.log("b");
+        a = 0         && console.log("c");
+        a = undefined && 2 * condition;
+        a = null      && condition + 3;
+        a = 2 * 3 - 6 && 4 - condition;
+        a = 10 == 7   && condition / 5;
+        a = !"string" && 6 % condition;
+        a = 0         && 7;
+
+        // don't compress these
+
+        a = condition        && true;
+        a = console.log("a") && 2;
+        a = 4 - condition    && "string";
+        a = 6 << condition   && -4.5;
+
+        a = condition        && false;
+        a = console.log("b") && NaN;
+        a = console.log("c") && 0;
+        a = 2 * condition    && undefined;
+        a = condition + 3    && null;
+
+    }
+    expect: {
+        var a;
+
+        a = condition;
+        a = console.log("a");
+        a = 2 * condition;
+        a = condition + 3;
+        a = 4 - condition;
+        a = condition / 5;
+        a = 6 << condition;
+        a = 7;
+
+        a = false;
+        a = NaN;
+        a = 0;
+        a = void 0;
+        a = null;
+        a = 0;
+        a = false;
+        a = false;
+        a = 0;
+
+        a = condition        && true;
+        a = console.log("a") && 2;
+        a = 4 - condition    && "string";
+        a = 6 << condition   && -4.5;
+
+        a = condition        && false;
+        a = console.log("b") && NaN;
+        a = console.log("c") && 0; 
+        a = 2 * condition    && void 0;
+        a = condition + 3    && null;
+    }
+}
+
+conditional_or: {
+    options = {
+        conditionals: true,
+        evaluate    : true
+    };
+    input: {
+        var a;
+        // compress these
+
+        a = true     || condition;
+        a = 1        || console.log("a");
+        a = 2 * 3    || 2 * condition;
+        a = 5 == 5   || condition + 3;
+        a = "string" || 4 - condition;
+        a = 5 + ""   || condition / 5;
+        a = -4.5     || 6 << condition;
+        a = 6        || 7;
+
+        a = false     || condition;
+        a = 0         || console.log("b");
+        a = NaN       || console.log("c");
+        a = undefined || 2 * condition;
+        a = null      || condition + 3;
+        a = 2 * 3 - 6 || 4 - condition;
+        a = 10 == 7   || condition / 5;
+        a = !"string" || 6 % condition;
+        a = null      || 7;
+
+        a = console.log(undefined && condition || null);
+        a = console.log(undefined || condition && null);
+
+        // don't compress these
+
+        a = condition        || true;
+        a = console.log("a") || 2;
+        a = 4 - condition    || "string";
+        a = 6 << condition   || -4.5;
+
+        a = condition        || false;
+        a = console.log("b") || NaN;
+        a = console.log("c") || 0;
+        a = 2 * condition    || undefined;
+        a = condition + 3    || null;
+
+    }
+    expect: {
+        var a;
+
+        a = true;
+        a = 1;
+        a = 6;
+        a = true;
+        a = "string";
+        a = "5";
+        a = -4.5;
+        a = 6;
+
+        a = condition;
+        a = console.log("b");
+        a = console.log("c");
+        a = 2 * condition;
+        a = condition + 3;
+        a = 4 - condition;
+        a = condition / 5;
+        a = 6 % condition;
+        a = 7;
+
+        a = console.log(null);
+        a = console.log(condition && null);
+
+        a = condition        || true;
+        a = console.log("a") || 2;
+        a = 4 - condition    || "string";
+        a = 6 << condition   || -4.5;
+
+        a = condition        || false;
+        a = console.log("b") || NaN;
+        a = console.log("c") || 0;
+        a = 2 * condition    || void 0;
+        a = condition + 3    || null;
+    }
+}
+