posix standard to overwrite redirection causes unexpected output

Question

I've been trying to build a small bash script to override some configuration in Node. I'm using zsh, so there's a warning if you redirect output in an existing file (see here). I've been trying the proposed answer, although the output is quite unusual. I was wondering maybe I'm doing something wrong here.

echo 'module.exports = { "foo": 1 }' > foo.js

node -e '
  const foo = require("./foo")
  foo.bar = 1
  console.log(`module.exports = ${JSON.stringify(foo)}`)
'

# output is: module.exports = {"foo":1,"bar":1}

node -e '
  const foo = require("./foo")
  foo.bar = 1
  console.log(`module.exports = ${JSON.stringify(foo)}`)
' > bar.js

cat bar.js

# output is: module.exports = {"foo":1,"bar":1}

node -e '
  const foo = require("./foo")
  foo.bar = 1
  console.log(`module.exports = ${JSON.stringify(foo)}`)
' >| foo.js

# output is: module.exports = {"bar":1}

I was under the impression that >| would bypass the "overwrite file" warning from zsh, but it does compute a "different than expected" return value.

Actual: module.exports = {"bar":1}
Expected: module.exports = {"foo":1,"bar":1}

Where have gone the bytes of "foo:1"? Could somebody explain me what's happening?

Answer 1

When the output of a command is redirected with > or >| to a file, that file is first truncated (emptied), then the command is executed.

When you run your Node.js code in the last example, the file foo.js is empty, due to to redirection. This is why the original data from that file is not visible in the output.