flatten.js

'use strict';

export default function flatten(obj) {
    const pairs = Object.keys(obj).reduce((acc, key) => {
        const pair = [key, obj[key]];
        return acc.concat([pair]);
    }, []);

    return pairs.reduce((acc, [k, v]) => {
        switch (typeof v) {
            case 'object':
                const newObj = Object.keys(v).reduce((acc, key) => {
                    return Object.assign(acc, { [`${k}[${key}]`]: v[key] });
                }, {});
                return Object.assign(acc, flatten(newObj))
            default:
                return Object.assign(acc, { [k]: v })
        }
    }, {});
}

test.js

'use strict';

import flatten from './flatten.js';

const testData = [
    {
        expected: {"x[0]": "1", "x[1]": "2", "x[2]": "3"},
        arguments: {"x": ["1","2","3"]}
    },
    {
        expected: {"a[d][0]": 1, "a[d][1]": 2},
        arguments: {"a": {"d": [1, 2]}}
    },
    {
        expected: {"a[b]": 3, "a[c]": 2, "a[d][0]": 1, "a[d][1]": 2, "x[0]": "1", "x[1]": "2", "x[2]": "3"},
        arguments: {"a": {"b": 3, "c": 2, "d": [1, 2]}, "x": ["1", "2", "3"]}
    },
    {
        expected: {"a[b]": 3},
        arguments: {"a": {"b": 3}}
    }
]

function test(testData) {
    const test = testData.map(data => {
        const result = solution(data.arguments);
        return JSON.stringify(result) === JSON.stringify(data.expected);
    }).map(result => result ? '.' : 'F').join('');

    console.log(test);
}

test(testData); // => ....