fix(core): parse stdout in json() (#1505)

[shubhransh-gupta]

Fixes #1505

Problem

ProcessOutput.json() previously parsed this.stdall. When CLI commands write structured JSON to stdout while logging diagnostic/progress messages to stderr (e.g. Railway CLI via railway add --json), stdall contains interleaved text from both streams, causing SyntaxError: Unexpected token ... in JSON at position ....

Solution

ProcessOutput.json() now parses this.stdout directly:

json<T = any>(): T {
  return JSON.parse(this.stdout)
}

Checklist

• Setup: Set the latest Node.js LTS version.
• Build: Bundle and types build cleanly.
• Tests: Unit tests pass.
• Sign: Commits follow conventional commits spec.

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[antonmedv]

way to over complicated.

[shubhransh-gupta]

Simplified! Stripped out all option flags and types. ProcessOutput.json() now simply parses this.stdout directly.

[antongolub]

1. This is definitely a breaking change. stdall captures json errors as stderr. I acknowledge that some users might deliberately redirect stdout to stderr for their own reasons.
2. buffer() should be aligned too. But ProcessPromise.toString/toPrimitive relies on this.stdall, so the buffer() follows the contract. Here is the divergence point.

[shubhransh-gupta]

Thanks @antongolub, that makes complete sense regarding the contract alignment (toString(), buffer(), and text() all defaulting to stdall).

To avoid a breaking change while keeping the implementation minimal (avoiding the overcomplicated options API @antonmedv pointed out), would you prefer keeping stdall as the default and simply allowing the stream to be passed as a string parameter?

json<T = any>(from: 'stdout' | 'stderr' | 'stdall' = 'stdall'): T {
  return JSON.parse(this[from])
}

This:

1. Preserves 100% backward compatibility with stdall and stays consistent with buffer() / toString().
2. Solves ProcessOutput.json() parses combined stdout+stderr, breaking on CLIs that log progress to stderr #1505 directly for tools logging progress to stderr (await $....json('stdout')).
3. Stays dead simple (a single line without nested option objects).

Would this work for you?

[shubhransh-gupta]

Updated the implementation in commit 58c5236:

• defaults to 'stdout', solving ProcessOutput.json() parses combined stdout+stderr, breaking on CLIs that log progress to stderr #1505 when stderr contains diagnostic progress logs.
• Supports specifying the stream source: p.json('stdout'), p.json('stderr'), or p.json('stdall').
• Clean 1-line implementation (return JSON.parse(this[from])) without heavy options/objects.
• Updated documentation and unit tests for both ProcessPromise.json() and ProcessOutput.json() across stream targets. All formatting and size-limit checks pass.