How to check if object or var is traversable in Javascript?

Question

How can I check the object or variable is traversable in Javascript or jQuery?

An object or a variable which is traversable must works in for each like object.forEach() and $.each(object, callback) in jQuery.

Actually I want to validate it then use it in for each loops.

Update:

The solution which I looking for is something like this:

if(is_traversable($var)) {
    $var.forEach(function(value, key) {
        console.log(value);
    }
}

And you may give me an implementation of is_traversable() function.

There are no `for each` loops in JavaScript. Do you mean `for of`? — Bergi, Sep 05 '16 at 13:41
@Bergi There is forEach() method for arrays in Javascript. see more http://www.w3schools.com/jsref/jsref_forEach.asp — Milad Rahimi, Sep 05 '16 at 14:04
Sure, but that's just a method not a loop. And any object could have a method with that name. Or are you looking for arrays? Then you should ask for that. — Bergi, Sep 05 '16 at 14:09

Nina Scholz · Accepted Answer · 2021-03-13T22:28:21.970

2

For arrays, use Array.isArray() and for objects, you could first check not null and typeof is equal to 'object' or 'function'.

The wanted function for it:

var isTraversable = o => Array.isArray(o)
     || o !== null && ['function', 'object'].includes(typeof o);

console.log(isTraversable());               // false
console.log(isTraversable(undefined));      // false
console.log(isTraversable(null));           // false
console.log(isTraversable(''));             // false
console.log(isTraversable(0));              // false
console.log(isTraversable('a'));            // false
console.log(isTraversable(42));             // false
console.log(isTraversable(/./));            // false
console.log(isTraversable([]));             // true
console.log(isTraversable([1]));            // true
console.log(isTraversable({}));             // true
console.log(isTraversable({ a: /./ }));     // true
console.log(isTraversable(function () {})); // true
console.log(isTraversable(() => true));     // true

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Mar 13 '21 at 22:28

answered Sep 05 '16 at 13:25

Nina Scholz

376,160
25
347
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Similarly you can use typeof to detect the type. I believe arrays will list as objects though so use caution. – Liam MacDonald Sep 05 '16 at 13:26
but `null` is an object and not iterable. – Nina Scholz Sep 05 '16 at 13:28
@NinaScholz null is not an object. Apparently `typeof null` is a bug in ecmascript when it outputs 'object' – Flame Sep 05 '16 at 13:33
Actually, there was a proposal to make `typeof null == "null"`, which was rejected: http://wiki.ecmascript.org/doku.php?id=harmony%3atypeof_null – Gerardo Furtado Sep 05 '16 at 13:36
1

This is not traversable : {} and [] , so you need to check also the length or if there is anything inside the object after it is the correct type with for( var p in o ) { return true; }. A string is also traversable when it is not empty. An object can be also a primitive type like a boolean or a number. These are not traversable. – Codebeat Aug 22 '18 at 21:31
This answer is incorrect. Functions are traversable like any other object, but this solution returns false for them. – luawtf Mar 13 '21 at 19:57

score 0 · Answer 2 · answered Sep 05 '16 at 13:32

You can use the following and create appropriate functions / validations:

var myArray = ["This is a test"];
var myObject = {"test" : "This is a test"};

if( Object.prototype.toString.call( myArray ) === '[object Array]' ) {
    alert( 'Array!' );
}

if( Object.prototype.toString.call( myObject ) === '[object Object]' ) {
    alert( 'Object!' );
}

Check out this one : Object.toString

georg · Answer 3 · 2016-09-05T13:37:32.397

forEach and friends are implemented in a generic fashion, to be eligible, their argument

must support the getter operator ([])
and must have a numeric length property.

That is, any string and any object with .length will work:

isIterable = x => x && !isNaN(x.length);

Note that for..of is different from forEach, it explicitly requires an argument to have Symbol.iterator.

a = {
    length: 10,
    5: 555
};

[].forEach.call(a, x => console.log(x)); // fine

for(var x of a) console.log(x); // no way

nmanikiran · Answer 4 · 2016-09-05T13:51:19.957

0

items might be Array Object, variable

Example

items = [1,2,3,4]; or items = { name: 'abc', email: 'abc@xyz.com'}

var items;

if (items && items instanceof Array) { // if Array
  items.forEach(function(value) {
    console.log(value);
  });
} else if (items && items instanceof Object) { // if Object
  for (key in items) {
    console.log(items[keys]);
  }
} else { // if variable 
  console.log(items);
}

edited Sep 05 '16 at 13:51

answered Sep 05 '16 at 13:46

nmanikiran

2,866
15
19

How to check if object or var is traversable in Javascript?

4 Answers4