javascript check if dictionary

Question

I have a simple program like:

var a = {\'a\': 1, \'b\': 2}
console.log(a)
console.log(a instanceof Array)
console.log(a.constructor instanceof Array)


        

Answer 1

To be more rigorous you can use JSON. But it's not a performance & memory efficient solution:

function isJsonable(v) {
    try{
        return JSON.stringify(v) === JSON.stringify(JSON.parse(JSON.stringify(v)));
     } catch(e){
        /*console.error("not a dict",e);*/
        return false;
    }
}

So the answer can be: (note that it is an inefficient method and is recommended for test purposed only)

function isDict(v) {
    return !!v && typeof v==='object' && v!==null && !(v instanceof Array) && !(v instanceof Date) && isJsonable(v);
}

Answer 2

function isDict(v) {
    return typeof v==='object' && v!==null && !(v instanceof Array) && !(v instanceof Date);
}

Answer 3

I use the toString method in Object.prototype, This works like a charm in all the cases(Array,null,undefined etc).

var array_var=[1,2];
var dict_var={
'a':'hey',
'b':'hello'
};
console.log(Object.prototype.toString.call(dict_var) === '[object Object]');//returns true
console.log(Object.prototype.toString.call(array_var) === '[object Object]');//returns false  

In short the toString method is used to represent the object, for example the toString method for an array returns '[object Array]'

Answer 4

The structure {'a': 1, 'b': 2} is a Javascript object. It can be used sort of like a dictionary, but Javascript does not have an actual dictionary type.

console.log(typeof a);            // "object"
console.log(Array.isArray(a));    // false, because it's not an array

If you want to know if something is an array, then use:

Array.isArray(a)

If you want to know if something is an object, then use:

typeof a === "object"

But, you will have to be careful because an Array is an object too.

---

If you want to know if something is a plain object, you can look at what jQuery does to detect a plain object:

isPlainObject: function( obj ) {
    // Not plain objects:
    // - Any object or value whose internal [[Class]] property is not "[object Object]"
    // - DOM nodes
    // - window
    if ( jQuery.type( obj ) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
        return false;
    }

    // Support: Firefox <20
    // The try/catch suppresses exceptions thrown when attempting to access
    // the "constructor" property of certain host objects, ie. |window.location|
    // https://bugzilla.mozilla.org/show_bug.cgi?id=814622
    try {
        if ( obj.constructor &&
                !hasOwn.call( obj.constructor.prototype, "isPrototypeOf" ) ) {
            return false;
        }
    } catch ( e ) {
        return false;
    }

    // If the function hasn't returned already, we're confident that
    // |obj| is a plain object, created by {} or constructed with new Object
    return true;
},

Answer 5

Would that work ?

function isDictObj(obj: any) {
  try {
    const test = {...obj}
  } catch(err) {
    return false
  }
  return true
}

My logic is that if it is a dictionnary object (key:values), then using spread operator with {} should work, otherwise throw an exception

Answer 6

The simplest approach to check if something is a dictionary in Javascript in a way that will not also return true when given an array is:

if (a.constructor == Object) {
    // code here...
}

This was inspired by the answer here.