babel-generator for typescript assertions

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

@@ -162,6 +162,9 @@ export function TSNullKeyword() {
 export function TSNeverKeyword() {
   this.word("never");
 }
+export function TSAssertsKeyword() {
+  this.word("asserts");
+}
 
 export function TSThisType() {
   this.word("this");
@@ -197,11 +200,17 @@ export function TSTypeReference(node) {
 }
 
 export function TSTypePredicate(node) {
+  if (node.assertsModifier) {
+    this.print(node.assertsModifier);
+    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 {});