Follow up from https://github.com/garronej/github_actions_toolkit

src/tell_if_project_uses_npm_or_yarn.ts

@@ -0,0 +1,50 @@
+
+import fetch from "node-fetch";
+const urlJoin: typeof import("path").join = require("url-join");
+import { setOutputFactory } from "./outputHelper";
+
+import { getActionParamsFactory } from "./inputHelper";
+
+export const { getActionParams } = getActionParamsFactory({
+    "inputNameSubset": [
+        "owner",
+        "repo",
+        "branch"
+    ] as const
+});
+
+export type Params = ReturnType<typeof getActionParams>;
+
+type CoreLike = {
+    debug: (message: string) => void;
+};
+
+export const { setOutput } = setOutputFactory<"npm_or_yarn">();
+
+export async function action(
+    _actionName: "tell_if_project_uses_npm_or_yarn",
+    params: Params,
+    core: CoreLike
+): Promise<Parameters<typeof setOutput>[0]> {
+
+    core.debug(JSON.stringify(params));
+
+    const { owner, repo } = params;
+
+    const branch = params.branch.split("/").reverse()[0];
+
+    const npm_or_yarn = await fetch(
+        urlJoin(
+            "https://raw.github.com",
+            owner,
+            repo,
+            branch,
+            "yarn.lock"
+        )
+    ).then(res => res.status === 404 ? "npm" : "yarn")
+
+    core.debug(`Version on ${owner}/${repo}#${branch} is using ${npm_or_yarn}`);
+
+    return { npm_or_yarn };
+
+}