How to check if JavaScript object is JSON

Question

I have a nested JSON object that I need to loop through, and the value of each key could be a String, JSON array or another JSON object. Depending on the type of object, I need to carry out different operations. Is there any way I can check the type of the object to see if it is a String, JSON object or JSON array?

I tried using typeof and instanceof but both didn't seem to work, as typeof will return an object for both JSON object and array, and instanceof gives an error when I do obj instanceof JSON.

To be more specific, after parsing the JSON into a JS object, is there any way I can check if it is a normal string, or an object with keys and values (from a JSON object), or an array (from a JSON array)?

For example:

JSON

var data = "{'hi':
             {'hello':
               ['hi1','hi2']
             },
            'hey':'words'
           }";

Sample JavaScript

var jsonObj = JSON.parse(data);
var path = ["hi","hello"];

function check(jsonObj, path) {
    var parent = jsonObj;
    for (var i = 0; i < path.length-1; i++) {
        var key = path[i];
        if (parent != undefined) {
            parent = parent[key];
        }
    }
    if (parent != undefined) {
        var endLength = path.length - 1;
        var child = parent[path[endLength]];
        //if child is a string, add some text
        //if child is an object, edit the key/value
        //if child is an array, add a new element
        //if child does not exist, add a new key/value
    }
}

How do I carry out the object checking as shown above?

Answer 1

I'd check the constructor attribute.

e.g.

var stringConstructor = "test".constructor;
var arrayConstructor = [].constructor;
var objectConstructor = ({}).constructor;

function whatIsIt(object) {
    if (object === null) {
        return "null";
    }
    if (object === undefined) {
        return "undefined";
    }
    if (object.constructor === stringConstructor) {
        return "String";
    }
    if (object.constructor === arrayConstructor) {
        return "Array";
    }
    if (object.constructor === objectConstructor) {
        return "Object";
    }
    {
        return "don't know";
    }
}

var testSubjects = ["string", [1,2,3], {foo: "bar"}, 4];

for (var i=0, len = testSubjects.length; i < len; i++) {
    alert(whatIsIt(testSubjects[i]));
}

Edit: Added a null check and an undefined check.

Answer 2

You can use Array.isArray to check for arrays. Then typeof obj == 'string', and typeof obj == 'object'.

var s = 'a string', a = [], o = {}, i = 5;
function getType(p) {
    if (Array.isArray(p)) return 'array';
    else if (typeof p == 'string') return 'string';
    else if (p != null && typeof p == 'object') return 'object';
    else return 'other';
}
console.log("'s' is " + getType(s));
console.log("'a' is " + getType(a));
console.log("'o' is " + getType(o));
console.log("'i' is " + getType(i));

's' is string
'a' is array
'o' is object
'i' is other

Answer 3

An JSON object is an object. To check whether a type is an object type, evaluate the constructor property.

function isObject(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == Object;
}

The same applies to all other types:

function isArray(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == Array;
}

function isBoolean(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == Boolean;
}

function isFunction(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == Function;
}

function isNumber(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == Number;
}

function isString(obj)
{
    return obj !== undefined && obj !== null && obj.constructor == String;
}

function isInstanced(obj)
{
    if(obj === undefined || obj === null) { return false; }

    if(isArray(obj)) { return false; }
    if(isBoolean(obj)) { return false; }
    if(isFunction(obj)) { return false; }
    if(isNumber(obj)) { return false; }
    if(isObject(obj)) { return false; }
    if(isString(obj)) { return false; }

    return true;
}