doc: specify return type for tty.isatty()

[belochub]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[BridgeAR]

Lite-CI https://ci.nodejs.org/job/node-test-pull-request-lite-pipeline/3212/

[vsemozhetbyt]

Node.js Collaborators, please, add 👍 here if you approve fast-tracking.

[vsemozhetbyt]

Landed in 643a2fa
Thank you!

[silverwind]

FWFI, boolean is not entirely correct, it can also be undefined in some edge cases:

#26946
#2333

[BridgeAR]

@silverwind those issues seem to be related to isTTY but not to isatty()? Looking at the functions code, I am pretty certain it can only return a boolean and never undefined.

[silverwind]

@BridgeAR ah, you're actually correct. I assumed they use the same implementation underneath, but this seems not the case.

parent.js

require("child_process").exec("node child.js", (_, stdout) => process.stdout.write(stdout));

child.js

console.log(process.stdin.isTTY);
console.log(process.stdin.isRaw);
console.log(process.stdin.setRawMode);
console.log(process.stdout.columns);
console.log(require("tty").isatty(process.stdin));
console.log(require("tty").isatty(process.stdout));

node parent.js prints

undefined
undefined
undefined
undefined
false
false