How to convert a nested JavaScript object to a flat array?

Question

I've been coding at this solution for hours now and can't seem to make much progress anymore. It's a very difficult function to write. Here is my input:

var testObj = {
  'b': 'num',
  'a': {
    'd': 'num',
    'c': 'num'
  },
  'g': ['num', 'num', {
    'x': 'num',
    'y': 'num'
  }],
  'e': 'num'
};

And here's the output I'm trying to generate from above:

var flatTestObj = [
  ['b', 'num'],
  ['a', 'obj'],
  ['a', 'obj', 'd', 'num'],
  ['a', 'obj', 'c', 'num'],
  ['g', 'arr'],
  ['g', 'arr', 0, 'num'],
  ['g', 'arr', 1, 'num'],
  ['g', 'arr', 2, 'obj'],
  ['g', 'arr', 2, 'obj', 'x', 'num'],
  ['g', 'arr', 2, 'obj', 'y', 'num'],
  ['e', 'num']
];

(Later I'm going to alphabetize the keys too but I can handle that part on my own so I excluded that from the question to make it easier to answer) but yeah the point is to guarantee the same order traversal of objects of this format later on.

Basically, have it recursively go through the object and add each key/index mapping as a new element to flatTestObj so later you can iterate through flatTestObj and get every member from testObj by referencing every other element from the current flatTestObj index array value.

Here's what I have so far but I'm having difficulty getting it to work with nesting:

var flattened = [];

function flatten(json, acc) {
  var keys = Object.keys(json);

  for (var i = 0; i < keys.length; i++) {
    var key = keys[i];
    var val = json[key];

    var strVal = typeof val === 'object' ? 'obj' : val;

    if (acc.length === 0) {
      acc = [key, strVal];
    } else {
      acc = acc.concat([key, strVal]);
    }

    if (typeof val === 'object') {
      flatten(val, acc);
    } else {
      flattened.push(acc);
      acc = [];
    }
  }
}

flatten(testObj, []);

Rick Hitchcock · Accepted Answer · 2016-07-14 09:51:13Z

1

This gives the output you're looking for:

var flattened = [];
function flatten(json, acc, inArray) {
  var key;

  acc = acc || [];
  for(key in json) {
    if(inArray) {
      key = +key;  //coerce to number
    }
    if(json[key] instanceof Array) {
      flattened.push(acc.concat([key, 'arr']));
      flatten(json[key], acc.concat([key, 'arr']), true);
    }
    else if(json[key] instanceof Object) {
      flattened.push(acc.concat([key, 'obj']));
      flatten(json[key], acc.concat([key, 'obj']));
    }
    else {
      flattened.push(acc.concat(key, json[key]));
    }
  }
} //flatten

Essentially, you need to include acc.concat() each time you push to the flattened array.

var testObj = {
  'b': 'num',
  'a': {
    'd': 'num',
    'c': 'num'
  },
  'g': ['num', 'num', {
    'x': 'num',
    'y': 'num'
  }],
  'e': 'num'
};

var flattened = [];
function flatten(json, acc, inArray) {
  var key;
  
  acc = acc || [];
  for(key in json) {
    if(inArray) {
      key = +key;  //coerce to number
    }
    if(json[key] instanceof Array) {
      flattened.push(acc.concat([key, 'arr']));
      flatten(json[key], acc.concat([key, 'arr']), true);
    }
    else if(json[key] instanceof Object) {
      flattened.push(acc.concat([key, 'obj']));
      flatten(json[key], acc.concat([key, 'obj']));
    }
    else {
      flattened.push(acc.concat(key, json[key]));
    }
  }
} //flatten

flatten(testObj);
console.log(JSON.stringify(flattened));

edited Jul 14, 2016 at 9:51

answered Jul 13, 2016 at 22:57

Rick Hitchcock

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1 Comment

Chron Bag Over a year ago

Beautiful! So simple too! Thank you so much! I'm going to study this for a while but it looks perfect! Except for array indicies being set as strings in the flattened array but I think I can figure out how to fix that.

adeneo · Accepted Answer · 2016-07-13 23:28:19Z

1

Here's one way to do it with recursive function calls

var testObj = {
    'b': 'num',
    'a': {
        'd': 'num',
        'c': 'num'
    },
    'g': ['num', 'num', {
        'x': 'num',
        'y': 'num'
    }],
    'e': 'num'
};

var flatTestObj = (function it(what, arr, chain) {
    if (typeof what === "object") {
        var fn = function(k) {
            var type = Array.isArray(what[k]) ? 'arr' : 'obj' ,
            	vArr = [k, typeof what[k] === "object" ? type : what[k]],
                fArr = chain.concat(vArr);
                            
            arr.push(fArr); 
            it(what[k], arr, fArr);
        }

    	if (Array.isArray(what)) for (var i in what) fn(i)
        else Object.keys(what).forEach(fn);
    }
    return arr;
})(testObj, [], []);

document.body.innerHTML = '<pre>' + JSON.stringify(flatTestObj, 0, 4) + '</pre>';

edited Jul 13, 2016 at 23:28

answered Jul 13, 2016 at 23:01

adeneo

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1 Comment

Chron Bag Over a year ago

Eeek, looks like someone answered while you were making this. Hopefully it can help out someone. I upvoted you anyway!

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