fix: move run-script banners to stderr when in json mode

Fixes #7354

docs/bin/build.js

@@ -16,7 +16,7 @@ const run = require('../lib/build.js')
 const { paths } = require('../lib/index')
 
 run(paths)
-  .then((res) => console.log(`Wrote ${res.length} files`))
+  .then((res) => console.error(`Wrote ${res.length} files`))
   .catch((err) => {
     process.exitCode = 1
     console.error(err)

lib/utils/display.js

@@ -267,16 +267,22 @@ class Display {
         if (this.#outputState.buffering) {
           this.#outputState.buffer.push([level, meta, ...args])
         } else {
-          // HACK: if it looks like the banner and we are in a state where we hide the
-          // banner then dont write any output. This hack can be replaced with proc-log.META
-          const isBanner = args.length === 1 &&
-            typeof args[0] === 'string' &&
-            args[0].startsWith('\n> ') &&
-            args[0].endsWith('\n')
-          const hideBanner = this.#silent || ['exec', 'explore'].includes(this.#command)
-          if (!(isBanner && hideBanner)) {
-            this.#writeOutput(level, meta, ...args)
+          // HACK: Check if the argument looks like a run-script banner. This can be
+          // replaced with proc-log.META in @npmcli/run-script
+          if (typeof args[0] === 'string' && args[0].startsWith('\n> ') && args[0].endsWith('\n')) {
+            if (this.#silent || ['exec', 'explore'].includes(this.#command)) {
+              // Silent mode and some specific commands always hide run script banners
+              break
+            } else if (this.#json) {
+              // In json mode, change output to stderr since we dont want to break json
+              // parsing on stdout if the user is piping to jq or something.
+              // XXX: in a future (breaking?) change it might make sense for run-script to
+              // always output these banners with proc-log.output.error if we think they
+              // align closer with "logging" instead of "output"
+              level = output.KEYS.error
+            }
           }
+          this.#writeOutput(level, meta, ...args)
         }
         break
     }

smoke-tests/test/pack-json-output.js

@@ -0,0 +1,12 @@
+
+const t = require('tap')
+const setup = require('./fixtures/setup.js')
+
+t.test('pack --json returns only json on stdout', async t => {
+  const { npmLocal } = await setup(t)
+
+  const { stderr, stdout } = await npmLocal('pack', '--json')
+
+  t.match(stderr, /> npm@.* prepack\n/, 'stderr has banner')
+  t.ok(JSON.parse(stdout), 'stdout can be parsed as json')
+})