Use Record for objec types with a single indexer (#244)

Author: kevinbarabash

src/transform.js

@@ -817,7 +817,8 @@ const transform = {
         if (
           t.isTSSymbolKeyword(key) ||
           t.isTSStringKeyword(key) ||
-          t.isTSNumberKeyword(key)
+          t.isTSNumberKeyword(key) ||
+          t.isTSTypeReference(key)
         ) {
           elements.push(indexer);
         } else {
@@ -835,6 +836,37 @@ const transform = {
         }
       });
 
+      // If there's only one property and it's an indexer convert the object
+      // type to use Record, e.g.
+      // {[string]: number} -> Record<string, number>
+      if (
+        spreads.length === 0 &&
+        elements.length === 1 &&
+        indexers.length === 1
+      ) {
+        const indexer = indexers[0];
+        const value = indexer.typeAnnotation.typeAnnotation;
+        const key = indexer.parameters[0].typeAnnotation.typeAnnotation;
+
+        const record = t.tsTypeReference(
+          t.identifier("Record"),
+          t.tsTypeParameterInstantiation([key, value])
+        );
+
+        if (indexer.readonly) {
+          path.replaceWith(
+            t.tsTypeReference(
+              t.identifier("Readonly"),
+              t.tsTypeParameterInstantiation([record])
+            )
+          );
+        } else {
+          path.replaceWith(record);
+        }
+
+        return;
+      }
+
       if (spreads.length > 0 && elements.length > 0) {
         path.replaceWith(
           t.tsIntersectionType([...spreads, t.tsTypeLiteral(elements)])

test/fixtures/convert/historical-types/object-generic-type-annotation/ts.js

@@ -1,3 +1 @@
-let a: {
-  [key: string]: any;
-};
+let a: Record<string, any>;

test/fixtures/convert/object-types/anonymous-indexer/ts.js

@@ -1,3 +1 @@
-let obj: {
-  [key: number]: string;
-};
+let obj: Record<number, string>;

test/fixtures/convert/object-types/indexer01/ts.js

@@ -1,3 +1 @@
-let obj: {
-  [key: number]: string;
-};
+let obj: Record<number, string>;

test/fixtures/convert/object-types/indexer02/ts.js

@@ -1,2 +1,2 @@
 type Key = "foo" | "bar";
-let obj: { [key in Key]?: string };
+let obj: Record<Key, string>;

test/fixtures/convert/object-types/readonly-indexer/ts.js

@@ -1,3 +1 @@
-let obj: {
-  readonly [key: number]: string;
-};
+let obj: Readonly<Record<number, string>>;

test/fixtures/convert/object-types/type-reference-indexer/ts.js

@@ -1 +1 @@
-let obj: { [key in A]?: string };
+let obj: Record<A, string>;

test/fixtures/convert/object-types/writeonly-indexer/ts.js

@@ -1,3 +1 @@
-let obj: {
-  [key: number]: string;
-};
+let obj: Record<number, string>;