var fs = require('fs');

function Reader() {
    this.content = null;
}

Reader.prototype.getContent = function() {
    if (this.content === null) {
        return new Promise((resolve, reject) => {
            console.log('new promise');
            fs.readFile('file', 'utf8', function(err, contents) {
                if (err) return reject(err);
                this.content = contents;
                console.log('resolved content', this.content);
                resolve(this.content);
            });
        });
    }

    return Promise.resolve(this.content);
}

Reader.prototype.getContent2 = function() {
    if (this.content === null) {
        this.content = new Promise((resolve, reject) => {
            console.log('new promise');
            fs.readFile('file', 'utf8', function(err, contents) {
                if (err) return reject(err);
                this.content = contents;
                console.log('resolved content', this.content);
                resolve(this.content);
            });
        });
    }

    return this.content;
}

console.log('after calling readFile');

var r = new Reader();

Promise.all([r.getContent(), r.getContent()])
    .then((results) => {
        console.log(results);
    });

Promise.all([r.getContent2(), r.getContent2()])
    .then((results) => {
        console.log(results);
    });