when you have a json too big for memory, here's away to stream out specific keys you want

get-big-JSON-nested-item.js

/* this is a sort of filter function that returns every value that matches a provided path
if you know the path of the node you want, use the function in the other file--it'll be faster */

var JSONStream = require('JSONStream')
var fs = require('fs')
// use immutable for easier assigning of deeply nested props
var Immutable = require('immutable')
var writeJSON = require('simple-json-writing-util.js')

var M = Immutable.Map({})

// this will pass to its callback any value that has the path '/true/images', where `true` is a wildcard
// check JSONStream docs but i think regex also works
var stream = JSONStream.parse([true, 'images', { emitKey: true }], (value, fullPath) => {
  // fullPath = [resultsOfWildcard, 'images']
  // which matches the immutable `setIn` syntax
    M = M.setIn(fullPath, value)
})
var readStream = fs.createReadStream('./filtered.json')
readStream.pipe(stream)

readStream.on('end', () => {
    writeJSON('home-images.json', M.toJS())
})

get-specific-node-in-big-JSON.js

/* this will get a node at a specific path. it's faster than above function, but doesn't support wildcards in path  */

var fs = require('fs')
var DepthStream = require('json-depth-stream')
var zlib = require('zlib')

function getBigJSONKey(readPath, keyPath, writePath, gzipOutput = false) {
    var gunzip = zlib.createGunzip()
    var gzip = zlib.createGzip()
    var readStream = fs.createReadStream(readPath)
    var writeStream = fs.createWriteStream(writePath)
    const json = new DepthStream(depth)

    // decide depth of parse based on length of keypath provided
    const depth = keyPath.length

    // construct query portion of the pipe. will return chunk key of which matches `keyPath`
    const q = json.query(keyPath)
    q.once('end', () => {
        readStream.unpipe(json)
    })

    // decide write compression
    if (gzipOutput === true) {
        q.pipe(gzip)
            .pipe(writeStream)
    } else {
        q.pipe(writeStream)
    }

    // start read
    if (readPath.slice(-2) === "gz") {

        readStream
            .pipe(gunzip)
            .pipe(json)
    } else {
        readStream
            .pipe(json)

    }
}