How do I read a single character from stdin synchronously?

Question

Put in another way, what is the node.js equivalent of C's getchar function? (which waits for input and when it gets it, it returns the character code of the letter, and subsequent calls get more characters from stdin)

I tried searching google, but none of the answers were synchronous.

@Bulkan that only works for files being piped into the script. The accepted answer for that question will work better: stackoverflow.com/a/8452997/893780 — robertklep
– robertklep, Commented Nov 25, 2013 at 8:15
You need to set stdin.setRawMode(true); Have a look at this answer for a complete example: stackoverflow.com/a/12506613/771431 — xonya
– xonya, Commented Apr 26, 2019 at 16:57

Anthony Garcia-Labiad · Accepted Answer · 2020-10-06 23:54:42Z

2

Here is a simple implementation of getChar based fs.readSync:

fs.readSync(fd, buffer, offset, length)

Unlike the other answer, this will be synchronous, only read one char from the stdin and be blocking, just like the C's getchar:

let fs = require('fs')

function getChar() {
  let buffer = Buffer.alloc(1)
  fs.readSync(0, buffer, 0, 1)
  return buffer.toString('utf8')
}

console.log(getChar())
console.log(getChar())
console.log(getChar())

answered Oct 6, 2020 at 23:54

Anthony Garcia-Labiad

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3 Comments

Jianwu Chen Over a year ago

WIth this method, I got following error, any idea? Error: EAGAIN: resource temporarily unavailable, read at Object.readSync (node:fs:611:3)

Anthony Garcia-Labiad Over a year ago

Looks like stdin is not available - I'm assuming you're testing this code in interactive mode (with node -i)? It will not work when executed like this, but will work when doing node script.js or echo "input" | node script.js.

dbkk Over a year ago

For me, this indeed reads one char, but only after the user pressed "enter" and stdin contains the entire line. Is there a way to get a char right after the user presses it?

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