Implement cat in JavaScript

Author: Konvaly

implement-shell-tools/cat/cat.mjs

@@ -0,0 +1,58 @@
+import process from "node:process";
+import { promises as fs } from "node:fs";
+import { parseArgs } from "node:util";
+
+const { values, positionals } = parseArgs({
+  options: {
+    number: { type: "boolean", short: "n", default: false },
+    "number-nonblank": { type: "boolean", short: "b", default: false },
+  },
+  allowPositionals: true,
+});
+
+const numberAll = values.number;
+const numberNonBlank = values["number-nonblank"];
+
+if (positionals.length === 0) {
+  console.error("Usage: node cat.js [-n] [-b] <path>...");
+  process.exit(1);
+}
+
+let lineNumber = 1;
+
+for (const path of positionals) {
+  let content;
+  try {
+    content = await fs.readFile(path, "utf-8");
+  } catch {
+    console.error(`cat: ${path}: No such file or directory`);
+    process.exitCode = 1;
+    continue;
+  }
+
+  if (!numberAll && !numberNonBlank) {
+    process.stdout.write(content);
+    continue;
+  }
+
+  const lines = content.split("\n");
+  const endsWithNewline = lines[lines.length - 1] === "";
+  if (endsWithNewline) {
+    lines.pop();
+  }
+
+  for (let i = 0; i < lines.length; i++) {
+    const line = lines[i];
+    const isLastLine = i === lines.length - 1;
+    const lineEnding = !isLastLine || endsWithNewline ? "\n" : "";
+
+    if (numberNonBlank && line === "") {
+      process.stdout.write(lineEnding);
+    } else {
+      process.stdout.write(
+        `${String(lineNumber).padStart(6)}\t${line}${lineEnding}`,
+      );
+      lineNumber++;
+    }
+  }
+}