Late to the party, but this solution I have made can handle recursivity, and nested array/object
function is(className, object) {
    return Object.prototype.toString.call(object) === '[object '+ className +']';
}
const DataEncoder = function() {
    this.levels = [];
}
DataEncoder.prototype.__dataEncoding = function(data, actualKey = null) {
    let finalString = '';
    if (actualKey) {
        this.levels.push(actualKey);
    }
    const keys = Object.keys(data);
    const l = keys.length;
    for(let a = 0; a < l; a++) {
        const key = keys[a];
        let value = data[key];
        if (is('Object', value)) {
            finalString += this.__dataEncoding(value, key);
        } else if (is('Array', value)) {
            const arrValSize = value.length;
            for (let b = 0; b < arrValSize; b++) {
                let arrVal = value[b];
                if (actualKey) {
                    finalString += actualKey;
                    for(let c = 1; c < this.levels.length; c++) finalString += '[' + this.levels[c] + ']';
                    if (arrVal === undefined || arrVal === null) arrVal = '';
                    finalString += '[' + key + '][]=' + arrVal + '&';
                } else {
                    if (arrVal === undefined || arrVal === null) arrVal = '';
                    finalString += key + '[]=' + arrVal + '&';
                }
            }
        } else {
            if (actualKey) {
                finalString += this.levels[0];
                for(let c = 1; c < this.levels.length; c++) finalString += '[' + this.levels[c] + ']';
                if (value === undefined || value === null) value = '';
                finalString += '[' + key + ']=' + value + '&';
            } else {
                if (value === undefined || value === null) value = '';
                finalString += key + '=' + value + '&';
            }
        }
    }
    this.levels.pop();
    return finalString;
}
DataEncoder.prototype.encode = function(data) {
    if (!is('Object', data) || data === {}) return null;
    return this.__dataEncoding(data).slice(0, -1);
}
Usage:
const testData = {
  name: "John",
  age: 13,
  skills: ['games', 'programming', 'reading', 'singing'],
  invests: {
    education: [120.3, 50.5],
    kids: 70,
    optical: {
      north: 20.5,
      south: 10.70,
      west: 6,
      east: [7]
    },
    deeper: {
      first: {
        landing: 5
      }
    }
  }
};
const encoder = new DataEncoder();
encoder.encode(testData);
Result:
name=John&age=13&skills[]=games&skills[]=programming&skills[]=reading&skills[]=singing&invests[education][]=120.3&invests[education][]=50.5&invests[kids]=70&invests[optical][north]=20.5&invests[optical][south]=10.7&invests[optical][west]=6&optical[optical][east][]=7&invests[deeper][first][landing]=5
I know that it needs encodeURIComponent method, but can be added easily
EDIT, IMPROVEMENTS
function is(className, object) {
    return Object.prototype.toString.call(object) === '[object '+ className +']';
}
const DataEncoder = function() {
    this.levels = [];
    this.actualKey = null;
}
DataEncoder.prototype.__dataEncoding = function(data) {
    let uriPart = '';
    const levelsSize = this.levels.length;
    if (levelsSize) {
      uriPart = this.levels[0];
      for(let c = 1; c < levelsSize; c++) {
        uriPart += '[' + this.levels[c] + ']';
      }
    }
    let finalString = '';
    if (is('Object', data)) {
        const keys = Object.keys(data);
        const l = keys.length;
        for(let a = 0; a < l; a++) {
            const key = keys[a];
            let value = data[key];
            this.actualKey = key;
            this.levels.push(this.actualKey);
            finalString += this.__dataEncoding(value);
        }
    } else if (is('Array', data)) {
        if (!this.actualKey) throw new Error("Directly passed array does not work")
        const aSize = data.length;
        for (let b = 0; b < aSize; b++) {
            let aVal = data[b];
            this.levels.push(b);
            finalString += this.__dataEncoding(aVal);
        }
    } else {
        finalString += uriPart + '=' + encodeURIComponent(data) + '&';
    }
    this.levels.pop();
    return finalString;
}
DataEncoder.prototype.encode = function(data) {
    if (!is('Object', data) || !Object.keys(data).length) return null;
    return this.__dataEncoding(data).slice(0, -1);
}
Now it can handle any deep, with nested array/objects, this edit has the same usage
const testData = {
  name: "John",
  age: 13,
  skills: ['games', 'programming', 'reading', 'singing'],
  invests: {
    education: [120.3, 50.5],
    kids: 70,
    optical: {
      north: 20.5,
      south: 10.70,
      west: 6,
      east: [7]
    },
    deeper: {
      first: {
        landing: 5,
        arrayLike: [
          {
            despite: true,
            superb: 'yes',
            omg: {
              kiss: ['la'],
              maybe: {
                thiss: {
                  wont: {
                    work: 'false'
                  }
                }
              }
            },
            incredible: ['lalolanda', 'raidcall', 'phase', [5], [{waw: '@wesome'}]],
          }
        ]
      }
    }
  }
};
const encoder = new DataEncoder();
encoder.encode(testData);
Result:
name=John&age=13&skills[0]=games&skills[1]=programming&skills[2]=reading&skills[3]=singing&invests[education][0]=120.3&invests[education][1]=50.5&invests[kids]=70&invests[optical][north]=20.5&invests[optical][south]=10.7&invests[optical][west]=6&invests[optical][east][0]=7&invests[deeper][first][landing]=5&invests[deeper][first][arrayLike][0][despite]=true&invests[deeper][first][arrayLike][0][superb]=yes&invests[deeper][first][arrayLike][0][omg][kiss][0]=la&invests[deeper][first][arrayLike][0][omg][maybe][thiss][wont][work]=false&invests[deeper][first][arrayLike][0][incredible][0]=lalolanda&invests[deeper][first][arrayLike][0][incredible][1]=raidcall&invests[deeper][first][arrayLike][0][incredible][2]=phase&invests[deeper][first][arrayLike][0][incredible][3][0]=5&invests[deeper][first][arrayLike][0][incredible][4][0][waw]=%40wesome