Node.js Promise reference error

• You forgot to capitalize Promise
• Your code structure is terrible
• Wrap legacy node async code in Promises
• Defer error handling code to caller
• import native node libs at root level (make them consts too)

Here is the refactored version:

const fs = require("fs");
const util = require("util");
const read = util.promisify(fs.readFile);

function init() {
    var reads = ["employees.json", "department.json"]
                .map(file => read(`./data/${file}`, "utf8").then(JSON.parse));

    return Promise.all(reads)
           .then(([employees, departments]) => ({ employees, departments }));
}

init().then(console.log).catch(console.error);

Function check is not defined in the scope of getAllEmployees function, because you have defined check function as local function inside initialize function.

First weird thing:

function initialize(){
    var fs = require("fs");

Maybe it's best if you set all require modules at the top, not inside the function.

I did some modifications to your code, now you won't have that error:

var employees = [];
var departments = [];
var error = 0;
var fs = require("fs");

function initialize(){
    fs.readFile("./data/employees.json", 'utf8', function(err, data){
        if(err){
            error = 1;
        }
        employees = JSON.parse(data);
        //console.log(employees);
    });


    fs.readFile("./data/department.json", 'utf8', function(err, data){
        if(err){
            error = 1;
        }
        departments = JSON.parse(data);
       // console.log(departments);
    });
}

function check() {
    return new promise(function(resolve,reject){
        if (error === 0){
            resolve("Success");
        }
        else if(error === 1){
           reject("unable to read file");
        }
    })     
};

function getAllEmployees(){
    initialize();
    check().then(function(employees){
        console.log(employees);
    }).catch(function(){
        console.log("No results returned");
    });
}

Summary of changes I did:

First, locate the main problem about you are asking, why are you getting "check function is not found"?. As I said in the beginning, check function was declared in the initialize scope function and getAllEmployees was declared outside so the main error is clear.

What I did: change initialize function to only get json data and set two global variables: employees and departments. Move check function to outside of initialize scope, as a module function to check promises.

Change getAllEmployees function to first, call initialize (to get json data and set the global variables) and call check function.

There are best ways to manage promises as for example the way provide as @Rafael in his answer.

The explicit construction is an antipattern. If you have Node.js 8 you can use promisify or use bluebird version of with an older environment. With it you can rewrite your code in much cleaner way and handle the errors properly.

Also it make sense to say that its a bad practice to throw(reject with) string

const {promisify} = require('util');

const fs = require('fs');
const readFileAsync = promisify(fs.readFile);

Promise.all([
    readFileAsync("./data/employees.json", {encoding: 'utf8'}).then(x => JSON.parse(x)),
    readFileAsync("./data/department.json", {encoding: 'utf8'}).then(x => JSON.parse(x))
])
    .then(([employees, departments]) => {
        console.log(employees);
    })
    .catch(console.log)