How to detect if a variable is a pure javascript object

Question

I have a function I need to pass an object to. I use typeof operator to make a check before processing. But looking at this link, it appears that many javascript instances, such as array or regex, are typed as Objects.

I need my argument to be a pure object (like this : {key: value, . . .}).

Is their any way I can check if a variable is pure object, without having to run specific test for each Object instance, like Array.isArray() ?

Answer 1

To achieve expected result, use below option of finding constructor name to check if variable is pure javascript Object or not

As per MDN,

All objects (with the exception of objects created with Object.create(null)) will have a constructor property. Objects created without the explicit use of a constructor function (i.e. the object and array literals) will have a constructor property that points to the Fundamental Object constructor type for that object.

Please refer this link for more details on constructor property - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor

var x = {a:1,b:2};
var y = [1,2,3];

console.log(x.constructor.name === "Object")//x.constructor.name is Object
console.log(y.constructor.name === "Object")//y.constructor.name is Array

Answer 2

You can check prototypes:

function isPureObject(input) {
  return null !== input && 
    typeof input === 'object' &&
    Object.getPrototypeOf(input).isPrototypeOf(Object);
}

console.log(isPureObject({}));               // true
console.log(isPureObject(new Object()));     // true
console.log(isPureObject(undefined));        // false
console.log(isPureObject(null));             // false
console.log(isPureObject(1));                // false
console.log(isPureObject('a'));              // false
console.log(isPureObject(false));            // false
console.log(isPureObject([]));               // false
console.log(isPureObject(new Array()));      // false
console.log(isPureObject(() => {}));         // false
console.log(isPureObject(function () {}));   // false

Answer 3

If you just want to check for "normal" pure objects (the ones created by {...}, JSON.parse(...), etc.), then this function will work

function is_pure_object(val) {
   return val ? Object.getPrototypeOf(val)==Object.prototype : false
}

If you also need to handle objects without a prototype (very rare, only created by Object.create(null)), then you need to do an additional check:

function is_pure_object2(val) {
   if (!val)
      return false
   let proto = Object.getPrototypeOf(val)
   return proto == Object.prototype || proto == null
}

Notes about other solutions:

• Checking val.constructor is not reliable:
  • fails on values like {constructor: 1}
  • fails on Object.create(null)
  • error if Object.prototype.constructor is modified
• Object.getPrototypeOf(val).isPrototypeOf(Object) is also not ideal:
  • error on Object.create(null)
  • error if Object.prototype.isPrototypeOf is modified

(Object.prototype being modified is unlikely, but I've dealt with it before)

Answer 4

the shortest version

const isObj = o => o?.constructor === Object;

found here: https://stackoverflow.com/a/61684890/7138254

Answer 5

Detection of a plain object is a bit problematic because of the definition of "plain" or "pure". I use the following method to detect plain objects. I never use x instanceof NodeList type validations because they do not work on cross-frame situations. (each window or frame has its own instance)

I think this is the simplest and most effective way of detecting plain objects.

function isPlainObject(o) {    
    var c = Object.prototype.toString.call(o) == '[object Object]'
        && o.constructor && o.constructor.name=="Object";
    return c === true;
}

function myTestFunction(){
/* ... */
}

class myTestClass{
/* ... */
}
        
console.log( isPlainObject({"a":1,"b":2}) ); //true
console.log( isPlainObject({}) ); //true
console.log( isPlainObject(null) ); //false
console.log( isPlainObject("my string") ); //false
console.log( isPlainObject("") ); //false
console.log( isPlainObject([1,2,3]) ); //false
console.log( isPlainObject(document.querySelectorAll("*")) ); //false
console.log( isPlainObject(new RegExp(/[a-z]+/)) ); //false
console.log( isPlainObject(myTestFunction) ); //false
console.log( isPlainObject(new myTestFunction()) ); //false
console.log( isPlainObject(myTestClass) ); //false
console.log( isPlainObject(new myTestClass()) ); //false

Answer 6

Using the lodash library, you can achieve this with the following function:

const isPureObject = _.isPlainObject({ foo: 'Zebra' });
expect(isPureObject).to.be.true;
    
const isMapPureObject = _.isPlainObject(new Map());
expect(isMapPureObject).to.be.false;

The official docs can be found here.