Introduced tstyche for type checking and to guard against unintentional API changes. For example, a test like this would have caught #3006 (#3020)

Author: smallsaucepan

.monorepolint.config.mjs

@@ -17,6 +17,7 @@ const MAIN_PACKAGE = "@turf/turf";
 
 const TAPE_PACKAGES = []; // projects that have tape tests
 const TYPES_PACKAGES = []; // projects that have types tests
+const TSTYCHE_PACKAGES = []; // projects that use tstyche for type tests.
 const BENCH_PACKAGES = []; // projects that have benchmarks
 
 // iterate all the packages and figure out what buckets everything falls into
@@ -39,6 +40,10 @@ glob.sync(path.join(__dirname, "packages", "turf-*")).forEach((pk) => {
   if (fs.existsSync(path.join(pk, "types.ts"))) {
     TYPES_PACKAGES.push(name);
   }
+
+  if (fs.existsSync(path.join(pk, "test/types.tst.ts"))) {
+    TSTYCHE_PACKAGES.push(name);
+  }
 });
 
 const TS_TAPE_PACKAGES = TAPE_PACKAGES.filter(
@@ -216,6 +221,15 @@ export default {
       includePackages: TYPES_PACKAGES,
     }),
 
+    packageScript({
+      options: {
+        scripts: {
+          "test:types": "tstyche",
+        },
+      },
+      includePackages: TSTYCHE_PACKAGES,
+    }),
+
     requireDependency({
       options: {
         devDependencies: {
@@ -242,6 +256,15 @@ export default {
       includePackages: TS_PACKAGES,
     }),
 
+    requireDependency({
+      options: {
+        devDependencies: {
+          tstyche: "^6.2.0",
+        },
+      },
+      includePackages: TSTYCHE_PACKAGES,
+    }),
+
     requireDependency({
       options: {
         dependencies: {

packages/turf-line-offset/package.json

@@ -54,7 +54,7 @@
     "docs": "tsx ../../scripts/generate-readmes.ts",
     "test": "pnpm run /test:.*/",
     "test:tape": "tsx test.ts",
-    "test:types": "tsc --esModuleInterop --module node16 --moduleResolution node16 --noEmit --strict types.ts"
+    "test:types": "tstyche"
   },
   "devDependencies": {
     "@turf/truncate": "workspace:*",
@@ -63,6 +63,7 @@
     "benchmark": "^2.1.4",
     "load-json-file": "^7.0.1",
     "tape": "^5.9.0",
+    "tstyche": "^6.2.0",
     "tsup": "^8.4.0",
     "tsx": "^4.19.4",
     "typescript": "^5.8.3",

packages/turf-line-offset/test/types.tst.ts

@@ -0,0 +1,37 @@
+import { lineOffset } from "../index.js";
+import { lineString, multiLineString } from "@turf/helpers";
+import type { Feature, LineString, MultiLineString } from "geojson";
+
+import { expect } from "tstyche";
+
+const line = lineString([
+  [0, 0],
+  [10, 10],
+]);
+const multiLine = multiLineString([
+  [
+    [0, 0],
+    [10, 10],
+  ],
+  [
+    [5, 5],
+    [15, 15],
+  ],
+]);
+
+/**
+ * If the syntax below starts generating errors it's possible you've narrowed
+ * the input arguments which is likely to be a breaking change.
+ */
+expect(lineOffset).type.toBeCallableWith(line, 50);
+expect(lineOffset).type.toBeCallableWith(line.geometry, 50);
+expect(lineOffset).type.toBeCallableWith(multiLine, 50);
+expect(lineOffset).type.toBeCallableWith(multiLine.geometry, 50);
+expect(lineOffset).type.toBeCallableWith(line, 50, { units: "miles" });
+
+/**
+ * If the sytax in this section starts generating errors, it's possible you've
+ * broadened the return type which is likely to be a breaking change.
+ */
+expect(lineOffset(line, 50)).type.toBe<Feature<LineString>>();
+expect(lineOffset(multiLine, 50)).type.toBe<Feature<MultiLineString>>();