Read JSON from standard input and writes formatted JSON to standard output. Requires Node.js.

formatjson.js

#!/usr/bin/env node

// Reads JSON from stdin and writes equivalent
// nicely-formatted JSON to stdout.

var stdin = process.stdin,
    stdout = process.stdout,
    inputChunks = [];

stdin.resume();
stdin.setEncoding('utf8');

stdin.on('data', function (chunk) {
    inputChunks.push(chunk);
});

stdin.on('end', function () {
    var inputJSON = inputChunks.join(),
        parsedData = JSON.parse(inputJSON),
        outputJSON = JSON.stringify(parsedData, null, '    ');
    stdout.write(outputJSON);
    stdout.write('\n');
});

WARNING: it inserts "," between the chunks.

to fix this replace

    var inputJSON = inputChunks.join(),

with

    var inputJSON = inputChunks.join(""),

@Roam-Cooper, I think the join approach is better as it potentially optimizes the concatenation.

Using the double quotes in the join()
also fixed this error I was getting:

'SyntaxError: Unexpected token , in JSON at position 65536'

Nodejs readline native utility can be helpful to split the lines if you are streaming line by line.

'is what i am saying

Thanks for gist 💪.

Here's a terser version that uses some of the included fixes:

let inputJson = ''
process.stdin.setEncoding('utf8')
process.stdin.on('data', function (chunk) {
  inputJson += chunk
})
process.stdin.on('end', function () {
  const parsedData = JSON.parse(inputJson)
  process.stdout.write(JSON.stringify(parsedData))
})

Yet another updated and terser version, ready to edit and run!

function readStdin() {
  return new Promise(resolve => {
    let buf = ''
    process.stdin.setEncoding('utf8')
    process.stdin.on('data', chunk => (buf += chunk))
    process.stdin.on('end', () => resolve(buf))
  })
}

async function main() {
  const d = JSON.parse(await readStdin())
  // do stuff with d, then...
  console.log(JSON.stringify(d, null, 2))
}

Here's a oneliner that does the same thing (at least in mac and linux land, not sure about windows):

node -p 'JSON.stringify(JSON.parse(fs.readFileSync(0)),null,2)'

notably, JSON.parse can work off of a buffer and fs.readFileSync(0) reads the zero file descriptor, which is standard input.
Then, node -p is a way to execute and log the output from a statement. You could also write it with a node -e 'console.log(...)' if you would rather be in control of when or how the logging happens.