diff --git a/packages/babel-plugin-transform-classes/src/transformClass.js b/packages/babel-plugin-transform-classes/src/transformClass.js
index b4283eedf8..b6276f1444 100644
--- a/packages/babel-plugin-transform-classes/src/transformClass.js
+++ b/packages/babel-plugin-transform-classes/src/transformClass.js
@@ -688,26 +688,9 @@ export default function transformClass(
       classState.staticPropBody.map(fn => fn(t.cloneNode(classState.classRef))),
     );
 
-    const strictParent = path.findParent(path => {
-      if (path.isProgram() && path.node.sourceType === "module") {
-        return true;
-      }
-
-      if (path.isClassBody()) {
-        return true;
-      }
-
-      if (!path.isProgram() && !path.isBlockStatement()) {
-        return false;
-      }
-
-      return path.node.directives.some(
-        directive => directive.value.value === "use strict",
-      );
-    });
-
+    const isStrict = path.isInStrictMode();
     let constructorOnly = classState.classId && body.length === 1;
-    if (constructorOnly && !strictParent) {
+    if (constructorOnly && !isStrict) {
       for (const param of classState.construct.params) {
         // It's illegal to put a use strict directive into the body of a function
         // with non-simple parameters for some reason. So, we have to use a strict
@@ -720,7 +703,7 @@ export default function transformClass(
     }
 
     const directives = constructorOnly ? body[0].body.directives : [];
-    if (!strictParent) {
+    if (!isStrict) {
       directives.push(t.directive(t.directiveLiteral("use strict")));
     }
 
diff --git a/packages/babel-traverse/src/path/introspection.js b/packages/babel-traverse/src/path/introspection.js
index f3053343fe..2ab558d5aa 100644
--- a/packages/babel-traverse/src/path/introspection.js
+++ b/packages/babel-traverse/src/path/introspection.js
@@ -439,3 +439,33 @@ export function isConstantExpression() {
 
   return false;
 }
+
+export function isInStrictMode() {
+  const start = this.isProgram() ? this : this.parentPath;
+
+  const strictParent = start.find(path => {
+    if (path.isProgram({ sourceType: "module" })) return true;
+
+    if (path.isClass()) return true;
+
+    if (!path.isProgram() && !path.isFunction()) return false;
+
+    if (
+      path.isArrowFunctionExpression() &&
+      !path.get("body").isBlockStatement()
+    ) {
+      return false;
+    }
+
+    let { node } = path;
+    if (path.isFunction()) node = node.body;
+
+    for (const directive of node.directives) {
+      if (directive.value.value === "use strict") {
+        return true;
+      }
+    }
+  });
+
+  return !!strictParent;
+}
diff --git a/packages/babel-traverse/test/introspection.js b/packages/babel-traverse/test/introspection.js
new file mode 100644
index 0000000000..bdb953d102
--- /dev/null
+++ b/packages/babel-traverse/test/introspection.js
@@ -0,0 +1,103 @@
+import traverse from "../lib";
+import { parse } from "babylon";
+
+function getPath(code, options = { sourceType: "script" }) {
+  const ast = parse(code, options);
+  let path;
+  traverse(ast, {
+    Program: function(_path) {
+      path = _path;
+      _path.stop();
+    },
+  });
+  return path;
+}
+
+describe("path/introspection", function() {
+  describe("isInStrictMode", function() {
+    describe("classes", function() {
+      it("returns parent's strictness for class", function() {
+        let program = getPath("class Test extends Super {}");
+        let klass = program.get("body.0");
+        expect(klass.isInStrictMode()).toBeFalsy();
+
+        program = getPath(`"use strict"; class Test extends Super {}`);
+        klass = program.get("body.0");
+        expect(klass.isInStrictMode()).toBeTruthy();
+      });
+
+      it("returns true for class id", function() {
+        const program = getPath("class Test extends Super {}");
+        const id = program.get("body.0.id");
+        expect(id.isInStrictMode()).toBeTruthy();
+      });
+
+      it("returns true for superClass", function() {
+        const program = getPath("class Test extends Super {}");
+        const superClass = program.get("body.0.superClass");
+        expect(superClass.isInStrictMode()).toBeTruthy();
+      });
+
+      it("returns true for method", function() {
+        const program = getPath("class Test { test() {} }");
+        const method = program.get("body.0.body.body.0");
+        expect(method.isInStrictMode()).toBeTruthy();
+      });
+    });
+
+    describe("program", function() {
+      describe("when script", function() {
+        it("returns true when strict", function() {
+          let program = getPath(`test;`);
+          expect(program.isInStrictMode()).toBeFalsy();
+
+          program = getPath(`"use strict";`);
+          expect(program.isInStrictMode()).toBeTruthy();
+        });
+      });
+
+      describe("when module", function() {
+        it("returns true", function() {
+          const program = getPath(`test;`, { sourceType: "module" });
+          expect(program.isInStrictMode()).toBeTruthy();
+        });
+      });
+    });
+
+    describe("function", function() {
+      it("returns parent's strictness for function", function() {
+        let program = getPath("function test() {}");
+        let fn = program.get("body.0");
+        expect(fn.isInStrictMode()).toBeFalsy();
+
+        program = getPath(`function test() {"use strict";}`);
+        fn = program.get("body.0");
+        expect(fn.isInStrictMode()).toBeFalsy();
+
+        program = getPath(`"use strict"; function test() {}`);
+        fn = program.get("body.0");
+        expect(fn.isInStrictMode()).toBeTruthy();
+      });
+
+      it("returns function's strictness for id", function() {
+        let program = getPath("function test(a) {}");
+        let id = program.get("body.0.id");
+        expect(id.isInStrictMode()).toBeFalsy();
+
+        program = getPath(`function test(a) {"use strict";}`);
+        id = program.get("body.0.id");
+        expect(id.isInStrictMode()).toBeTruthy();
+      });
+
+      it("returns function's strictness for parameters", function() {
+        let program = getPath("function test(a) {}");
+        let param = program.get("body.0.params.0");
+        expect(param.isInStrictMode()).toBeFalsy();
+
+        program = getPath(`function test(a) {"use strict";}`);
+        param = program.get("body.0.params.0");
+        expect(param.isInStrictMode()).toBeTruthy();
+      });
+    });
+  });
+});