add assertions signature for TypeScript

packages/babel-generator/src/generators/typescript.js

@@ -197,11 +197,17 @@ export function TSTypeReference(node) {
 }
 
 export function TSTypePredicate(node) {
+  if (node.assertsModifier) {
+    this.word("asserts");
+    this.space();
+  }
   this.print(node.parameterName);
-  this.space();
-  this.word("is");
-  this.space();
-  this.print(node.typeAnnotation.typeAnnotation);
+  if (node.typeAnnotation) {
+    this.space();
+    this.word("is");
+    this.space();
+    this.print(node.typeAnnotation.typeAnnotation);
+  }
 }
 
 export function TSTypeQuery(node) {

packages/babel-generator/test/fixtures/typescript/arrow-function-assertion/input.js

@@ -0,0 +1,2 @@
+(x: any): asserts x => true;
+(x: any): asserts x is boolean => true;

packages/babel-generator/test/fixtures/typescript/arrow-function-assertion/output.js

@@ -0,0 +1,3 @@
+(x: any): asserts x => true;
+
+(x: any): asserts x is boolean => true;

packages/babel-generator/test/fixtures/typescript/function-assertion/input.js

@@ -0,0 +1,2 @@
+function f(x: any): asserts x {}
+(function(x: any): asserts x is boolean {})

packages/babel-generator/test/fixtures/typescript/function-assertion/output.js

@@ -0,0 +1,3 @@
+function f(x: any): asserts x {}
+
+(function (x: any): asserts x is boolean {});

packages/babel-parser/src/plugins/typescript/index.js

@@ -904,21 +904,34 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         const t: N.TsTypeAnnotation = this.startNode();
         this.expect(returnToken);
 
+        const assertsModifier = this.tsTryParse(
+          this.tsParseTypePredicateAssertsModifier.bind(this),
+        );
+
         const typePredicateVariable =
           this.tsIsIdentifier() &&
           this.tsTryParse(this.tsParseTypePredicatePrefix.bind(this));
 
         if (!typePredicateVariable) {
-          return this.tsParseTypeAnnotation(/* eatColon */ false, t);
+          if (!assertsModifier) {
+            // : type
+            return this.tsParseTypeAnnotation(/* eatColon */ false, t);
+          }
+
+          // : asserts foo
+          const node = this.startNodeAtNode(t);
+          node.parameterName = this.parseIdentifier();
+          node.assertsModifier = assertsModifier;
+          t.typeAnnotation = this.finishNode(node, "TSTypePredicate");
+          return this.finishNode(t, "TSTypeAnnotation");
         }
 
+        // : foo is type
         const type = this.tsParseTypeAnnotation(/* eatColon */ false);
-
-        const node: N.TsTypePredicate = this.startNodeAtNode(
-          typePredicateVariable,
-        );
+        const node = this.startNodeAtNode(t);
         node.parameterName = typePredicateVariable;
         node.typeAnnotation = type;
+        node.assertsModifier = assertsModifier;
         t.typeAnnotation = this.finishNode(node, "TSTypePredicate");
         return this.finishNode(t, "TSTypeAnnotation");
       });
@@ -946,6 +959,23 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
+    tsParseTypePredicateAssertsModifier(): boolean {
+      if (!this.tsIsIdentifier()) {
+        return false;
+      }
+
+      const id = this.parseIdentifier();
+      if (
+        id.name !== "asserts" ||
+        this.hasPrecedingLineBreak() ||
+        !this.tsIsIdentifier()
+      ) {
+        return false;
+      }
+
+      return true;
+    }
+
     tsParseTypeAnnotation(
       eatColon = true,
       t: N.TsTypeAnnotation = this.startNode(),

packages/babel-parser/test/fixtures/typescript/arrow-function/predicate-types/output.json

@@ -73,12 +73,12 @@
             },
             "typeAnnotation": {
               "type": "TSTypePredicate",
-              "start": 10,
+              "start": 8,
               "end": 21,
               "loc": {
                 "start": {
                   "line": 1,
-                  "column": 10
+                  "column": 8
                 },
                 "end": {
                   "line": 1,

packages/babel-parser/test/fixtures/typescript/assert-predicate/arrow-function/input.ts

@@ -0,0 +1,3 @@
+const assert1 = (value: unknown): asserts value is string => {}
+const assert2 = (value: unknown): asserts value => {}
+const assert3 = (value: unknown): asserts => {}