1 files changed, 55 insertions, 0 deletions
diff --git a/packages/idb-bridge/src/util/extractKey.ts b/packages/idb-bridge/src/util/extractKey.ts
new file mode 100644
index 000000000..fd14c5a64
--- /dev/null
+++ b/packages/idb-bridge/src/util/extractKey.ts
@@ -0,0 +1,55 @@
+import { Key, KeyPath, Value } from "./types";
+import valueToKey from "./valueToKey";
+
+// http://www.w3.org/TR/2015/REC-IndexedDB-20150108/#dfn-steps-for-extracting-a-key-from-a-value-using-a-key-path
+const extractKey = (keyPath: KeyPath, value: Value) => {
+  if (Array.isArray(keyPath)) {
+    const result: Key[] = [];
+
+    for (let item of keyPath) {
+      // This doesn't make sense to me based on the spec, but it is needed to pass the W3C KeyPath tests (see same
+      // comment in validateKeyPath)
+      if (
+        item !== undefined &&
+        item !== null &&
+        typeof item !== "string" &&
+        (item as any).toString
+      ) {
+        item = (item as any).toString();
+      }
+      result.push(valueToKey(extractKey(item, value)));
+    }
+
+    return result;
+  }
+
+  if (keyPath === "") {
+    return value;
+  }
+
+  let remainingKeyPath: string | null = keyPath;
+  let object = value;
+
+  while (remainingKeyPath !== null) {
+    let identifier;
+
+    const i = remainingKeyPath.indexOf(".");
+    if (i >= 0) {
+      identifier = remainingKeyPath.slice(0, i);
+      remainingKeyPath = remainingKeyPath.slice(i + 1);
+    } else {
+      identifier = remainingKeyPath;
+      remainingKeyPath = null;
+    }
+
+    if (!object.hasOwnProperty(identifier)) {
+      return;
+    }
+
+    object = object[identifier];
+  }
+
+  return object;
+};
+
+export default extractKey;