FakeJsonGen.js

const comma = ",";
const empty = "";
const cursor = ["object", "array", "none"];

class FakeJsonGen {
  constructor() {
    this.out = ""
    this.timebase = new Date().getTime();
    this.timeInterval = 5000;

    this.hasKey = false;
    this.hasValue = false;

    this.cursor = cursor[2];

    this.buffer = "";
  }

  setKVStatus = (k, v) => {
    this.hasKey = k;
    this.hasValue = v;
  }

  startObject = () => {
    if (this.out.slice(-1) === "}" || this.out.slice(-1) === "]") {
      this.buffer = this.buffer.concat(`${comma}`);
    }
    this.buffer = this.buffer.concat("{");
    return this;
  }

  endObject = () => {
    this.buffer = this.buffer.concat("}");
    this.setKVStatus(false, false);

    this.out = this.out.concat(this.buffer);
    this.buffer = "";
    return this;
  }

  startArray = () => {
    this.buffer = this.buffer.concat("[");
    return this;
  }

  setArrayValue = (value) => {
    if (typeof value === Number) {
      this.buffer = this.buffer.concat(value);
    } else {
      this.buffer = this.buffer.concat(`"${value}"`);
    }
    return this;
  }

  endArray = () => {
    this.buffer = this.buffer.concat("]");
    return this;
  }
    
  attachKV = (key, value) => {
    if (typeof value === 'object') {
      if (value.hasOwnProperty("buffer")) {
        this.buffer = this.buffer.concat(`"${key}":"${value.buffer}"`);
      }
    } else {
      this.buffer = this.buffer.concat(`"${key}":"${value}"`);
    }
    return this;
  }

  setKey = (key) => {
    if (!this.hasKey) { 
      this.buffer = this.buffer.concat(`"${key}":`);
      this.setKVStatus(true, false);
    }
    return this;
  }

  setValue = (value) => {
    if (this.hasKey && !this.hasValue) {
      this.buffer = this.buffer.concat(`"${value}"`);
      this.setKVStatus(true, true);
    }
    return this;
  }

  returnOut = () => {
    console.log(this.out);
  }
}

const fake = new FakeJsonGen();

fake.startObject();
fake.attachKV("foo", fake.startObject());
fake.attachKV("bar", "foobar");
fake.endObject().endObject();

fake.returnOut();

Gist for creating Fake JSON file