Javascript dynamically initialize nested map with some array values

Question

I am trying to generate a map of the following format dynamically based on user inputs:

{
  Jane: {
    Comments: ["Hello", "Hi"]
  },
  John: {
    Age: "999",
    Comments: "Hi"
  }
}

The keys, the values and the nests are all in runtime - so initially all I will know is that the top-level structure is a map. I attempted to make this at runtime using the below code.

var nest = function(map, keys, v) {
    if (keys.length === 1) {
          map[keys[0]] = v;
    } else {
      var key = keys.shift();
      map[key] = nest(typeof map[key] === 'undefined' ? {} : map[key], keys, v);
    }

    return map;
};

var persons = new Map();
// Usage 
nest(persons, ['John', 'Comments'], 'Hi');
nest(persons, ['John', 'Age'], '999');

nest(persons, ['Jane', 'Comments'], 'Wow');
nest(persons, ['Jane', 'Comments'], 'Hello');

console.log(persons);

However, it overwrites the value of Comments instead of making it as arrays. Can someone please help me with creating this non-overwriting nested map with array values? (Note: any other values except comments are not arrays)

Thanks in advance.

First off you can't access Map properties using bracket notation you need to use the set and get methods. Perhaps you meant to use an object? — pilchard
– pilchard, Commented Oct 20, 2021 at 21:58

Unmitigated · Accepted Answer · 2021-10-20 22:02:06Z

2

You can use Array#reduce to get the nested Map, after which you can set the key.

var nest = function(map, keys, v) {
    const lastKey = keys.pop(), innerMap = keys.reduce((acc, key)=>{
      if(!acc.has(key)) acc.set(key, new Map);
      return map.get(key);
    }, map);
    if(lastKey !== 'Comments') innerMap.set(lastKey, v);
    else {
      if(!innerMap.has(lastKey)) innerMap.set(lastKey, []);
      innerMap.get(lastKey).push(v);
    }
};

var persons = new Map();
// Usage 
nest(persons, ['John', 'Comments'], 'Hi');
nest(persons, ['John', 'Age'], '999');

nest(persons, ['Jane', 'Comments'], 'Wow');
nest(persons, ['Jane', 'Comments'], 'Hello');

console.log(persons); // Check browser console

answered Oct 20, 2021 at 22:02

Unmitigated

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