I have a (nested) data structure containing objects and arrays. How can I extract the information, i.e. access a specific or multiple values (or keys)?

For example:

var data = {
    code: 42,
    items: [{
        id: 1,
        name: 'foo'
    }, {
        id: 2,
        name: 'bar'
    }]
};

How could I access the name of the second item in items?

2 upvote
  flag
At least you shouldn't talk about "a JSON" if you're not talking about the JSON interchange format. This is just an object in literal notation. – Marcel Korpel
12 upvote
  flag
@Marcel: It has to be read as "I have a data nested data structure or JSON, how can I access a specific value?". I know the difference, but many people don't and might be searching for "JSON" rather than "object". Many questions actually are of the form "how can I access X in this JSON". The only place where I mention JSON in my answer is where I explain what it is. If you have a suggestion how to communicate this in a better way, I'm all ears. – Felix Kling
upvote
  flag
possible duplicate of JSON find in JavaScript – Travis J
upvote
  flag
@Felix Kling:i read this..its too effective fr me.So thank u very much – robins

16 Answers 11

up vote 762 down vote accepted

Preliminaries

JavaScript has only one data type which can contain multiple values: Object. An Array is a special form of object.

(Plain) Objects have the form

{key: value, key: value, ...}

Arrays have the form

[value, value, ...]

Both arrays and objects expose a key -> value structure. Keys in an array must be numeric, whereas any string can be used as key in objects. The key-value pairs are also called the "properties".

Properties can be accessed either using dot notation

const value = obj.someProperty;

or bracket notation, if the property name would not be a valid JavaScript identifier name [spec], or the name is the value of a variable:

// the space is not a valid character in identifier names
const value = obj["some Property"];

// property name as variable
const name = "some Property";
const value = obj[name];

For that reason, array elements can only be accessed using bracket notation:

const value = arr[5]; // arr.5 would be a syntax error

// property name / index as variable
const x = 5;
const value = arr[x];

Wait... what about JSON?

JSON is a textual representation of data, just like XML, YAML, CSV, and others. To work with such data, it first has to be converted to JavaScript data types, i.e. arrays and objects (and how to work with those was just explained). How to parse JSON is explained in the question Parse JSON in JavaScript? .

Further reading material

How to access arrays and objects is fundamental JavaScript knowledge and therefore it is advisable to read the MDN JavaScript Guide, especially the sections



Accessing nested data structures

A nested data structure is an array or object which refers to other arrays or objects, i.e. its values are arrays or objects. Such structures can be accessed by consecutively applying dot or bracket notation.

Here is an example:

const data = {
    code: 42,
    items: [{
        id: 1,
        name: 'foo'
    }, {
        id: 2,
        name: 'bar'
    }]
};

Let's assume we want to access the name of the second item.

Here is how we can do it step-by-step:

As we can see data is an object, hence we can access its properties using dot notation. The items property is accessed as follows:

data.items

The value is an array, to access its second element, we have to use bracket notation:

data.items[1]

This value is an object and we use dot notation again to access the name property. So we eventually get:

const item_name = data.items[1].name;

Alternatively, we could have used bracket notation for any of the properties, especially if the name contained characters that would have made it invalid for dot notation usage:

const item_name = data['items'][1]['name'];

I'm trying to access a property but I get only undefined back?

Most of the time when you are getting undefined, the object/array simply doesn't have a property with that name.

const foo = {bar: {baz: 42}};
console.log(foo.baz); // undefined

Use console.log or console.dir and inspect the structure of object / array. The property you are trying to access might be actually defined on a nested object / array.

console.log(foo.bar.baz); // 42

What if the property names are dynamic and I don't know them beforehand?

If the property names are unknown or we want to access all properties of an object / elements of an array, we can use the for...in [MDN] loop for objects and the for [MDN] loop for arrays to iterate over all properties / elements.

Objects

To iterate over all properties of data, we can iterate over the object like so:

for (const prop in data) {
    // `prop` contains the name of each property, i.e. `'code'` or `'items'`
    // consequently, `data[prop]` refers to the value of each property, i.e.
    // either `42` or the array
}

Depending on where the object comes from (and what you want to do), you might have to test in each iteration whether the property is really a property of the object, or it is an inherited property. You can do this with Object#hasOwnProperty [MDN].

As alternative to for...in with hasOwnProperty, you can use Object.keys [MDN] to get an array of property names:

Object.keys(data).forEach(function(prop) {
  // `prop` is the property name
  // `data[prop]` is the property value
});

Arrays

To iterate over all elements of the data.items array, we use a for loop:

for(let i = 0, l = data.items.length; i < l; i++) {
    // `i` will take on the values `0`, `1`, `2`,..., i.e. in each iteration
    // we can access the next element in the array with `data.items[i]`, example:
    // 
    // var obj = data.items[i];
    // 
    // Since each element is an object (in our example),
    // we can now access the objects properties with `obj.id` and `obj.name`. 
    // We could also use `data.items[i].id`.
}

One could also use for...in to iterate over arrays, but there are reasons why this should be avoided: Why is 'for(var item in list)' with arrays considered bad practice in JavaScript?.

With the increasing browser support of ECMAScript 5, the array method forEach [MDN] becomes an interesting alternative as well:

data.items.forEach(function(value, index, array) {
    // The callback is executed for each element in the array.
    // `value` is the element itself (equivalent to `array[index]`)
    // `index` will be the index of the element in the array
    // `array` is a reference to the array itself (i.e. `data.items` in this case)
}); 

In environments supporting ES2015 (ES6), you can also use the for...of [MDN] loop, which not only works for arrays, but for any iterable:

for (const item of data.items) {
   // `item` is the array element, **not** the index
}

In each iteration, for...of directly gives us the next element of the iterable, there is no "index" to access or use.


What if the "depth" of the data structure is unknown to me?

In addition to unknown keys, the "depth" of the data structure (i.e. how many nested objects) it has, might be unknown as well. How to access deeply nested properties usually depends on the exact data structure.

But if the data structure contains repeating patterns, e.g. the representation of a binary tree, the solution typically includes to recursively [Wikipedia] access each level of the data structure.

Here is an example to get the first leaf node of a binary tree:

function getLeaf(node) {
    if (node.leftChild) {
        return getLeaf(node.leftChild); // <- recursive call
    }
    else if (node.rightChild) {
        return getLeaf(node.rightChild); // <- recursive call
    }
    else { // node must be a leaf node
        return node;
    }
}

const first_leaf = getLeaf(root);

const root = {
    leftChild: {
        leftChild: {
            leftChild: null,
            rightChild: null,
            data: 42
        },
        rightChild: {
            leftChild: null,
            rightChild: null,
            data: 5
        }
    },
    rightChild: {
        leftChild: {
            leftChild: null,
            rightChild: null,
            data: 6
        },
        rightChild: {
            leftChild: null,
            rightChild: null,
            data: 7
        }
    }
};
function getLeaf(node) {
    if (node.leftChild) {
        return getLeaf(node.leftChild);
    } else if (node.rightChild) {
        return getLeaf(node.rightChild);
    } else { // node must be a leaf node
        return node;
    }
}

console.log(getLeaf(root).data);

A more generic way to access a nested data structure with unknown keys and depth is to test the type of the value and act accordingly.

Here is an example which adds all primitive values inside a nested data structure into an array (assuming it does not contain any functions). If we encounter an object (or array) we simply call toArray again on that value (recursive call).

function toArray(obj) {
    const result = [];
    for (const prop in obj) {
        const value = obj[prop];
        if (typeof value === 'object') {
            result.push(toArray(value)); // <- recursive call
        }
        else {
            result.push(value);
        }
    }
    return result;
}

const data = {
  code: 42,
  items: [{
    id: 1,
    name: 'foo'
  }, {
    id: 2,
    name: 'bar'
  }]
};


function toArray(obj) {
  const result = [];
  for (const prop in obj) {
    const value = obj[prop];
    if (typeof value === 'object') {
      result.push(toArray(value));
    } else {
      result.push(value);
    }
  }
  return result;
}

console.log(toArray(data));



Helpers

Since the structure of a complex object or array is not necessarily obvious, we can inspect the value at each step to decide how to move further. console.log [MDN] and console.dir [MDN] help us doing this. For example (output of the Chrome console):

> console.log(data.items)
 [ Object, Object ]

Here we see that that data.items is an array with two elements which are both objects. In Chrome console the objects can even be expanded and inspected immediately.

> console.log(data.items[1])
  Object
     id: 2
     name: "bar"
     __proto__: Object

This tells us that data.items[1] is an object, and after expanding it we see that it has three properties, id, name and __proto__. The latter is an internal property used for the prototype chain of the object. The prototype chain and inheritance is out of scope for this answer, though.

2 upvote
  flag
Some of what's being linked here is really asking how to do this in jQuery, which to be fair does simplify 1 or 2 things here. Not sure whether to make this more of a megapost or answer those separately - the basics covered here on what's an object what's an array are usually what's really being asked... . – Chris Moschini

You can access it this way

data.items[1].name

or

data["items"][1]["name"]

Both ways are equal.

upvote
  flag
Yea but you can't do data["items"].1.name – neaumusic
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  flag
First is far more intuitive, readable and shorter ;) I prefer using bracket property syntax only when the property name is variable. – DanteTheSmith

In case you're trying to access an item from the example structure by id or name, without knowing it's position in the array, the easiest way to do it would be to use underscore.js library:

var data = {
    code: 42,
    items: [{
        id: 1,
        name: 'foo'
    }, {
        id: 2,
        name: 'bar'
    }]
};

_.find(data.items, function(item) {
  return item.id === 2;
});
// Object {id: 2, name: "bar"}

From my experience, using higher order functions instead of for or for..in loops results in code that is easier to reason about, and hence more maintainable.

Just my 2 cents.

Using JSONPath would be one of the most flexible solutions if you are willing to include a library: https://github.com/s3u/JSONPath (node and browser)

For your use case the json path would be:

$..items[1].name

so:

var secondName = jsonPath.eval(data, "$..items[1].name");
upvote
  flag
Using eval() is not good solution. Instead first class function can be used. – pradeep gowda

At times, accessing a nested object using a string can be desirable. The simple approach is the first level, for example

var obj = { hello: "world" };
var key = "hello";
alert(obj[key]);//world

But this is often not the case with complex json. As json becomes more complex, the approaches for finding values inside of the json also become complex. A recursive approach for navigating the json is best, and how that recursion is leveraged will depend on the type of data being searched for. If there are conditional statements involved, a json search can be a good tool to use.

If the property being accessed is already known, but the path is complex, for example in this object

var obj = {
 arr: [
    { id: 1, name: "larry" },    
    { id: 2, name: "curly" },
    { id: 3, name: "moe" }
 ]
};

And you know you want to get the first result of the array in the object, perhaps you would like to use

var moe = obj["arr[0].name"];

However, that will cause an exception as there is no property of object with that name. The solution to be able to use this would be to flatten the tree aspect of the object. This can be done recursively.

function flatten(obj){
 var root = {};
 (function tree(obj, index){
   var suffix = toString.call(obj) == "[object Array]" ? "]" : "";
   for(var key in obj){
    if(!obj.hasOwnProperty(key))continue;
    root[index+key+suffix] = obj[key];
    if( toString.call(obj[key]) == "[object Array]" )tree(obj[key],index+key+suffix+"[");
    if( toString.call(obj[key]) == "[object Object]" )tree(obj[key],index+key+suffix+".");   
   }
 })(obj,"");
 return root;
}

Now, the complex object can be flattened

var obj = previous definition;
var flat = flatten(obj);
var moe = flat["arr[0].name"];//moe

Here is a jsFiddle Demo of this approach being used.

upvote
  flag
WTH would you want to use obj["arr[0].name"] instead of obj.arr[0].name? You hardly need/want to deal with flattened objects except for serialisation. – Bergi
upvote
  flag
@Bergi - I see this question commonly, and since this is being used canonically, I posted an answer to that version of it. If it is avoidable it is much faster to use obj.arr[0].name, but sometimes people want to pass string accessors around and this is an example of doing that. – Travis J
upvote
  flag
Urgh. Still, there's hardly a reason to flatten the complete object only to use a single string path, you could simply parse that and do a dynamic lookup. – Bergi

If you are looking for one or more objects that meets certain criteria you have a few options using query-js

//will return all elements with an id larger than 1
data.items.where(function(e){return e.id > 1;});
//will return the first element with an id larger than 1
data.items.first(function(e){return e.id > 1;});
//will return the first element with an id larger than 1 
//or the second argument if non are found
data.items.first(function(e){return e.id > 1;},{id:-1,name:""});

There's also a single and a singleOrDefault they work much like firstand firstOrDefaultrespectively. The only difference is that they will throw if more than one match is found.

for further explanation of query-js you can start with this post

upvote
  flag
I would love to know how this could be improved. Care to leave a comment? – Rune FS

I prefer JQuery. It's cleaner and easy to read.

 $.each($.parseJSON(data), function (key, value) {
    alert(value.<propertyname>);
});

This question is quite old, so as a contemporary update. With the onset of ES2015 there are alternatives to get a hold of the data you require. There is now a feature called object destructuring for accessing nested objects.

const data = {
  code: 42,
  items: [{
    id: 1,
    name: 'foo'
  }, {
    id: 2,
    name: 'bar'
  }]
};

const {
  items: [, {
    name: secondName
  }]
} = data;

console.log(secondName);

The above example creates a variable called secondName from the name key from an array called items, the lonely , says skip the first object in the array.

Notably it's probably overkill for this example, as simple array acccess is easier to read, but it comes in useful when breaking apart objects in general.

This is very brief intro to your specific use case, destructuring can be an unusual syntax to get used to at first. I'd recommend reading Mozilla's Destructuring Assignment documentation to learn more.

The Underscore js Way

Which is a JavaScript library that provides a whole mess of useful functional programming helpers without extending any built-in objects.

Solution:

var data = {
  code: 42,
  items: [{
    id: 1,
    name: 'foo'
  }, {
    id: 2,
    name: 'bar'
  }]
};

var item = _.findWhere(data.items, {
  id: 2
});
if (!_.isUndefined(item)) {
  console.log('NAME =>', item.name);
}

//using find - 

var item = _.find(data.items, function(item) {
  return item.id === 2;
});

if (!_.isUndefined(item)) {
  console.log('NAME =>', item.name);
}

Objects and arrays has a lot of built-in methods that can help you with processing data.

Note: in many of the examples I'm using arrow functions. They are similar to function expressions, but they bind the this value lexically.

Object.keys(), Object.values() (ES 2017) and Object.entries() (ES 2017)

Object.keys() returns an array of object's keys, Object.values() returns an array of object's values, and Object.entries() returns an array of object's keys and corresponding values in a format [key, value].

const obj = {
  a: 1
 ,b: 2
 ,c: 3
}

console.log(Object.keys(obj)) // ['a', 'b', 'c']
console.log(Object.values(obj)) // [1, 2, 3]
console.log(Object.entries(obj)) // [['a', 1], ['b', 2], ['c', 3]]

Object.entries() with a for-of loop and destructuring assignment

const obj = {
  a: 1
 ,b: 2
 ,c: 3
}

for (const [key, value] of Object.entries(obj)) {
  console.log(`key: ${key}, value: ${value}`)
}

It's very convenient to iterate the result of Object.entries() with a for-of loop and destructuring assignment.

For-of loop lets you iterate array elements. The syntax is for (const element of array) (we can replace const with var or let, but it's better to use const if we don't intend to modify element).

Destructuring assignment lets you extract values from an array or an object and assign them to variables. In this case const [key, value] means that instead of assigning the [key, value] array to element, we assign the first element of that array to key and the second element to value. It is equivalent to this:

for (const element of Object.entries(obj)) {
  const key = element[0]
       ,value = element[1]
}

As you can see, destructuring makes this a lot simpler.

Array.prototype.every() and Array.prototype.some()

The every() method returns true if the specified callback function returns true for every element of the array. The some() method returns true if the specified callback function returns true for some (at least one) element.

const arr = [1, 2, 3]

// true, because every element is greater than 0
console.log(arr.every(x => x > 0))
// false, because 3^2 is greater than 5
console.log(arr.every(x => Math.pow(x, 2) < 5))
// true, because 2 is even (the remainder from dividing by 2 is 0)
console.log(arr.some(x => x % 2 === 0))
// false, because none of the elements is equal to 5
console.log(arr.some(x => x === 5))

Array.prototype.find() and Array.prototype.filter()

The find() methods returns the first element which satisfies the provided callback function. The filter() method returns an array of all elements which satisfies the provided callback function.

const arr = [1, 2, 3]

// 2, because 2^2 !== 2
console.log(arr.find(x => x !== Math.pow(x, 2)))
// 1, because it's the first element
console.log(arr.find(x => true))
// undefined, because none of the elements equals 7
console.log(arr.find(x => x === 7))

// [2, 3], because these elements are greater than 1
console.log(arr.filter(x => x > 1))
// [1, 2, 3], because the function returns true for all elements
console.log(arr.filter(x => true))
// [], because none of the elements equals neither 6 nor 7
console.log(arr.filter(x => x === 6 || x === 7))

Array.prototype.map()

The map() method returns an array with the results of calling a provided callback function on the array elements.

const arr = [1, 2, 3]

console.log(arr.map(x => x + 1)) // [2, 3, 4]
console.log(arr.map(x => String.fromCharCode(96 + x))) // ['a', 'b', 'c']
console.log(arr.map(x => x)) // [1, 2, 3] (no-op)
console.log(arr.map(x => Math.pow(x, 2))) // [1, 4, 9]
console.log(arr.map(String)) // ['1', '2', '3']

Array.prototype.reduce()

The reduce() method reduces an array to a single value by calling the provided callback function with two elements.

const arr = [1, 2, 3]

// Sum of array elements.
console.log(arr.reduce((a, b) => a + b)) // 6
// The largest number in the array.
console.log(arr.reduce((a, b) => a > b ? a : b)) // 3

The reduce() method takes an optional second parameter, which is the initial value. This is useful when the array on which you call reduce() can has zero or one elements. For example, if we wanted to create a function sum() which takes an array as an argument and returns the sum of all elements, we could write it like that:

const sum = arr => arr.reduce((a, b) => a + b, 0)

console.log(sum([]))     // 0
console.log(sum([4]))    // 4
console.log(sum([2, 5])) // 7

A pythonic, recursive and functional approach to unravel arbitrary JSON trees:

handlers = {
    list:  iterate,
    dict:  delve,
    str:   emit_li,
    float: emit_li,
}

def emit_li(stuff, strong=False):
    emission = '<li><strong>%s</strong></li>' if strong else '<li>%s</li>'
    print(emission % stuff)

def iterate(a_list):
    print('<ul>')
    map(unravel, a_list)
    print('</ul>')

def delve(a_dict):
    print('<ul>')
    for key, value in a_dict.items():
        emit_li(key, strong=True)
        unravel(value)
    print('</ul>')

def unravel(structure):
    h = handlers[type(structure)]
    return h(structure)

unravel(data)

where data is a python list (parsed from a JSON text string):

data = [
    {'data': {'customKey1': 'customValue1',
           'customKey2': {'customSubKey1': {'customSubSubKey1': 'keyvalue'}}},
  'geometry': {'location': {'lat': 37.3860517, 'lng': -122.0838511},
               'viewport': {'northeast': {'lat': 37.4508789,
                                          'lng': -122.0446721},
                            'southwest': {'lat': 37.3567599,
                                          'lng': -122.1178619}}},
  'name': 'Mountain View',
  'scope': 'GOOGLE',
  'types': ['locality', 'political']}
]
upvote
  flag
This question is about JavaScript, not Python. Not sure whether there is an equivalent question for Python. – Felix Kling

I don't think questioner just only concern one level nested object, so I present the following demo to demonstrate how to access the node of deeply nested json object. All right, let's find the node with id '5'.

var data = {
  code: 42,
  items: [{
    id: 1,
    name: 'aaa',
    items: [{
        id: 3,
        name: 'ccc'
      }, {
        id: 4,
        name: 'ddd'
      }]
    }, {
    id: 2,
    name: 'bbb',
    items: [{
        id: 5,
        name: 'eee'
      }, {
        id: 6,
        name: 'fff'
      }]
    }]
};

var jsonloop = new JSONLoop(data, 'id', 'items');

jsonloop.findNodeById(data, 5, function(err, node) {
  if (err) {
    document.write(err);
  } else {
    document.write(JSON.stringify(node, null, 2));
  }
});
<script src="https://rawgit.com/dabeng/JSON-Loop/master/JSONLoop.js"></script>

Old question but as nobody mentioned lodash (just underscore).

In case you are already using lodash in your project, I think an elegant way to do this in a complex example:

Opt 1

_.get(response, ['output', 'fund', 'data', '0', 'children', '0', 'group', 'myValue'], '')

same as:

Opt 2

response.output.fund.data[0].children[0].group.myValue

The difference between the first and second option is that in the Opt 1 if you have one of the properties missing (undefined) in the path you don't get an error, it returns you the third parameter.

For array filter lodash has _.find() but I'd rather use the regular filter(). But I still think the above method _.get() is super useful when working with really complex data. I faced in the past really complex APIs and it was handy!

I hope it can be useful for who's looking for options to manipulate really complex data which the title implies.

You could use lodash _get function:

var object = { 'a': [{ 'b': { 'c': 3 } }] };

_.get(object, 'a[0].b.c');
// => 3

jQuery's grep function lets you filter through an array:

var data = {
    code: 42,
    items: [{
        id: 1,
        name: 'foo'
    }, {
        id: 2,
        name: 'bar'
    }]
};

$.grep(data.items, function(item) {
    if (item.id === 2) {
        console.log(item.id); //console id of item
        console.log(item.name); //console name of item
        console.log(item); //console item object
        return item; //returns item object
    }

});
// Object {id: 2, name: "bar"}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

Using lodash would be good solution

Ex:

var object = { 'a': { 'b': { 'c': 3 } } };                                                                                               
_.get(object, 'a.b.c');                                                                                             
// => 3  

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