denoland/deno · error · TypeError

Cannot get 'stdout': 'stdout' is not piped

Error message

Cannot get 'stdout': 'stdout' is not piped

What it means

Deno's ChildProcess only creates a .stdout ReadableStream when the Command was spawned with stdout: "piped". The private #stdout field stays null for "inherit", "null", or other stdio settings, and the getter throws a TypeError instead of returning null. Note that spawn() defaults stdout to "inherit", unlike output() which defaults to "piped".

Source

Thrown at ext/process/40_process.js:458

  [_stderrRid];

  #pid;
  get pid() {
    return this.#pid;
  }

  #stdin = null;
  get stdin() {
    if (this.#stdin == null) {
      throw new TypeError("Cannot get 'stdin': 'stdin' is not piped");
    }
    return this.#stdin;
  }

  #stdout = null;
  get stdout() {
    if (this.#stdout == null) {
      throw new TypeError("Cannot get 'stdout': 'stdout' is not piped");
    }
    return this.#stdout;
  }

  #stderr = null;
  get stderr() {
    if (this.#stderr == null) {
      throw new TypeError("Cannot get 'stderr': 'stderr' is not piped");
    }
    return this.#stderr;
  }

  constructor(key = null, {
    signal,
    rid,
    pid,
    stdinRid,
    stdoutRid,

View on GitHub (pinned to 9ad36f7a2c)

Solutions

1. Set stdout: "piped" in the Deno.Command options before calling spawn()
2. If you only need the finished output, skip spawn() and use await cmd.output() instead, which defaults stdout/stderr to "piped"
3. Guard access: only touch child.stdout when the options you passed had stdout: "piped"

Example fix

// before
const child = new Deno.Command("echo", { args: ["hi"] }).spawn();
const body = child.stdout; // TypeError: 'stdout' is not piped

// after
const child = new Deno.Command("echo", { args: ["hi"], stdout: "piped" }).spawn();
const body = child.stdout;

const opts = { stdout: "piped", ...rest }; // ensure piped before spawn
const wantsStdout = opts.stdout === "piped";
const child = new Deno.Command(cmd, opts).spawn();
if (wantsStdout) { const reader = child.stdout.getReader(); }

function isStdoutPiped(options: Deno.CommandOptions): boolean {
  return options?.stdout === "piped";
}

try { stream = child.stdout; } catch (err) { if (err instanceof TypeError && err.message.includes("not piped")) { /* skip capture */ } else throw err; }

When it happens

Trigger: Accessing child.stdout on a ChildProcess from new Deno.Command(cmd, opts).spawn() when opts.stdout is omitted (defaults to "inherit") or set to "inherit"/"null". For example: const child = new Deno.Command("echo", { args: ["hi"] }).spawn(); child.stdout;

Common situations: Porting code from the removed Deno.run() API; assuming spawn() defaults match output() defaults; helper functions that unconditionally read child.stdout regardless of how the caller configured stdio.

Related errors

• Cannot get 'stderr': 'stderr' is not piped
• Illegal constructor
• Cannot collect output: 'stdout' is locked
• Piped stdin is not supported for this function, use 'Deno.Co
• ERR_INVALID_ARG_VALUE

AI-assisted analysis of denoland/deno@9ad36f7a2c (2026-08-20). Data as JSON: /api/errors/185578e3c9499f3d. Report an issue: GitHub.