Bun 1.3.5 Adds Native PTY Support and Compile‑Time Dead‑Code Elimination

Bun 1.3.5 was released with a set of low‑level enhancements that are especially relevant for developers building CLI tools or aiming for minimal bundle sizes.

1. Native PTY Support: Can Bun Run Vim?

The release adds a built‑in Bun.Terminal API that creates and manages pseudo‑terminals (PTY). Previously, interactive tools like vim or htop required complex third‑party libraries or hacks because they depend on a TTY environment for colors, cursor movement, and user input.

With the new terminal option, a child process can be mounted to a PTY directly:

const commands = ["echo Hello from PTY!", "exit"];
const proc = Bun.spawn(["bash"], {
  terminal: {
    cols: 80,
    rows: 24,
    data(terminal, data) {
      process.stdout.write(data);
      if (data.includes("$")) {
        terminal.write(commands.shift() + "
");
      }
    },
  },
});
await proc.exited;
proc.terminal.close();

When a PTY is attached, process.stdout.isTTY becomes true, enabling:

Color output to render correctly.

Interactive prompts to work as expected.

Embedding a vim editor inside custom tools.

Note: PTY support currently works on Linux and macOS; Windows is not yet supported.

2. Compile‑time Feature Flags: Cutting Unused Code at Build Time

Bun now standardizes compile‑time feature flags. By importing feature from bun:bundle, developers can write branches that are resolved during bundling.

import { feature } from "bun:bundle";

if (feature("PREMIUM")) {
  // This code is included only when the PREMIUM flag is enabled
  initPremiumFeatures();
}

if (feature("DEBUG")) {
  // This code is removed entirely when DEBUG is disabled
  console.log("Debug mode");
}

Running bun build --feature=PREMIUM ./app.ts replaces feature("PREMIUM") with true and feature("DEBUG") with false. Combined with minification, the false branches are eliminated completely, leaving no dead code.

Smaller bundles : Unused code never reaches the final artifact.

Increased safety : Debug or internal test code cannot leak into production.

Greater flexibility : Enables easy A/B testing or separate free/paid builds.

3. Bun.stringWidth: Accurate Emoji Alignment

Unicode width calculation has long been problematic. The new implementation of Bun.stringWidth correctly treats complex emojis (e.g., 🇺🇸, 👋🏽, 👨‍👩‍👧) as occupying two character cells, handles zero‑width characters like soft hyphens, and properly ignores ANSI escape sequences.

Better emoji support : Complex emojis are measured as width 2 instead of inconsistent values.

Zero‑width character handling : Invisible characters such as Word Joiner are counted as width 0.

ANSI escape sequence handling : Color codes and hyperlink sequences no longer inflate string length.

This improvement is a boon for developers building CLI tables, progress bars, and other text‑based UI components.

4. Other Notable Improvements

V8 type‑check API : New C++ APIs like v8::Value::IsMap() and IsArray() improve native addon compatibility.

S3 upload support : The contentDisposition option lets the built‑in S3 client control whether a file is downloaded or previewed.

.npmrc variable expansion : Fixed issues where environment variables inside quotes were ignored, aligning behavior with native npm.

Numerous bug fixes : Includes fixes for high CPU usage on macOS, fetch proxy authentication, and WebSocket crashes on Windows.

Reference: Bun v1.3.5 Release Blog