JavaScript: How to validate a string is really a string

Question

I'm creating a framework where I'm giving developers the ability to create objects with custom types using defined metadata structures in XML:

<class name="Class">
    <attr name="id" type="Yuid" mandatory="true"/>
    <attr name="name"type="YString"/>
</class>

then in javascript/typescript you can do something like:

 const instance = new Class({
    name: "bob"
 });

The idea is to add validators to the class attribute generators to assert the XML schema is valid.

For simple primitives I was using the primitive constructors like str = new String(str) until we started running into some weird issues when displaying this data. To illustrate:

const thang = new String("thang");
const theng = String("theng");
const thing = "thing";
const thong = String({ rick: "sanchez" });
const thung = new String({ rick: "sanchez" });

console.log(thang, typeof thang); // [String: 'thang'] 'object
console.log(theng, typeof theng); // theng string
console.log(thing, typeof thing); // thing string
console.log(thong, typeof thong); // [object Object] string
console.log(thung, typeof thung); // [String: '[object Object]'] 'object'

console.log({}.toString(), typeof {}.toString()); // [object Object] string
console.log("abc".toString(), typeof "abc".toString()); // abc string
console.log([1, 2, 3].toString(), typeof [1, 2, 3].toString()); // 1,2,3 string
console.log((5).toString(), typeof (5).toString()); // 5 string

console.log(`thang is ${thang}`); // thang is thang
console.log(`theng is ${thang}`); // theng is theng
console.log(`thing is ${thang}`); // thing is thing
console.log(`thong is ${thong}`); // thong is [object Object]
console.log(`thung is ${thung}`); // thung is [object Object]

TypeScript hints are the same, I would be getting strings in all cases, but reality is a bit different so how can I be entirely sure than I can transform any value into string?

new String returns an object. To convert any value to a string, call String without new. — Felix Kling
– Felix Kling, Commented Mar 14, 2019 at 20:57
@FelixKling check what happened when i did thong = String({}) — Simon
– Simon, Commented Mar 14, 2019 at 20:59
Yes, that's the expected value when converting an object to a string. Do you expect/want something else? It's not quite clear to me what your question is about tbh. — Felix Kling
– Felix Kling, Commented Mar 14, 2019 at 20:59
I expect to log/display a string and see the value of the string, not [object Object] — Simon
– Simon, Commented Mar 14, 2019 at 21:00

NyTrOuS · Accepted Answer · 2019-03-14 21:00:42Z

1

Instead of using:

const thang = new String("thang");

use:

const thang = "thang";

answered Mar 14, 2019 at 21:00

NyTrOuS

732 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ld00d · Accepted Answer · 2019-03-14 21:07:41Z

0

In addition to the typeof check, you could check for the object's constructor's name

> var x = new String('hello')
> x
[String: 'hello']
> typeof x
'object'
> x.constructor.name
'String'

answered Mar 14, 2019 at 21:07

Ld00d

417 bronze badges

Collectives™ on Stack Overflow

JavaScript: How to validate a string is really a string

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related