feat: downlevel template literal type in index signatures

sandersn · Dec 2, 2022 · efcb682 · efcb682
1 parent c6cfd17
commit efcb682
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diff --git a/README.md b/README.md
@@ -294,6 +294,25 @@ type T = [/** foo */ number, /** bar */ string];
 The downlevel semantics are exactly the same as the original, but
 the TypeScript language service won't be able to show the member names.
 
+### `{ [key: T]: A }` (4.4)
+
+Typescript 4.4 supports template literals as index signature types:
+
+```ts
+type O = { [key: `ak${string}`]: A };
+```
+
+becomes
+
+```ts
+type O = { [key: string]: A };
+```
+
+#### Semantics
+
+The downlevel d.ts will be less strict because an index signature would
+have a more general type (`string`) instead of the original template literal type.
+
 ### `in out T` (4.7)
 
 Typescript 4.7 supports variance annotations on type parameter declarations:

diff --git a/baselines/reference/ts3.4/test.d.ts b/baselines/reference/ts3.4/test.d.ts
@@ -71,3 +71,6 @@ export declare const foo: {
 export type IR = IteratorResult<number>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts3.5/test.d.ts b/baselines/reference/ts3.5/test.d.ts
@@ -71,3 +71,6 @@ export declare const foo: {
 export type IR = IteratorResult<number>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts3.6/test.d.ts b/baselines/reference/ts3.6/test.d.ts
@@ -73,3 +73,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts3.7/test.d.ts b/baselines/reference/ts3.7/test.d.ts
@@ -73,3 +73,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts3.8/test.d.ts b/baselines/reference/ts3.8/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts3.9/test.d.ts b/baselines/reference/ts3.9/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts4.0/test.d.ts b/baselines/reference/ts4.0/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = string;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts4.1/test.d.ts b/baselines/reference/ts4.1/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts4.2/test.d.ts b/baselines/reference/ts4.2/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts4.3/test.d.ts b/baselines/reference/ts4.3/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: string]: number;
+};
diff --git a/baselines/reference/ts4.4/test.d.ts b/baselines/reference/ts4.4/test.d.ts
@@ -74,3 +74,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: `${string}abc${string}`]: number;
+};
diff --git a/baselines/reference/ts4.5/test.d.ts b/baselines/reference/ts4.5/test.d.ts
@@ -71,3 +71,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: `${string}abc${string}`]: number;
+};
diff --git a/baselines/reference/ts4.6/test.d.ts b/baselines/reference/ts4.6/test.d.ts
@@ -71,3 +71,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: `${string}abc${string}`]: number;
+};
diff --git a/baselines/reference/ts4.7/test.d.ts b/baselines/reference/ts4.7/test.d.ts
@@ -71,3 +71,6 @@ export declare const foo: {
 export type IR = IteratorResult<number, string>;
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+export type TTemplateLiteralIndexSignature = {
+    [key: `${string}abc${string}`]: number;
+};
diff --git a/index.js b/index.js
@@ -367,6 +367,26 @@ function doTransform(checker, targetVersion, k) {
         ts.unescapeLeadingUnderscores(member.name.escapedText),
         /*hasTrailingNewline*/ false
       );
+    } else if (
+        semver.lt(targetVersion, "4.4.0") &&
+        ts.isIndexSignatureDeclaration(n) &&
+        ts.SyntaxKind.TemplateLiteralType === n.parameters[0].type.kind
+    ) {
+      const [p] = n.parameters;
+      return ts.factory.createIndexSignature(
+        n.modifiers,
+        [
+          ts.factory.createParameterDeclaration(
+            p.modifiers,
+            p.dotDotDotToken,
+            p.name,
+            p.questionToken,
+            ts.factory.createKeywordTypeNode(ts.SyntaxKind.StringKeyword),
+            p.initializer
+          )
+        ],
+        n.type
+      );
     } else if (semver.lt(targetVersion, "4.7.0") && ts.isTypeParameterDeclaration(n)) {
       return ts.factory.createTypeParameterDeclaration(
         n.modifiers?.filter(

diff --git a/test/test.d.ts b/test/test.d.ts
@@ -78,3 +78,5 @@ export type IR = IteratorResult<number, string>;
 
 /** Template Literal - supported since 4.1 < should be StringKeyword */
 export type TTemplateLiteral = `${string}abc${string}`;
+
+export type TTemplateLiteralIndexSignature = { [key: `${string}abc${string}`]: number }
Original file line number	Diff line number	Diff line change
Expand Up		@@ -78,3 +78,5 @@ export type IR = IteratorResult<number, string>;

		/** Template Literal - supported since 4.1 < should be StringKeyword */
		export type TTemplateLiteral = `${string}abc${string}`;

		export type TTemplateLiteralIndexSignature = { [key: `${string}abc${string}`]: number }