TypeScript compiler not detecting variable type correctly

Question

In TypeScript, I'm attempting to use a for (let x in obj) loop but the TypeScript compiler does not recognise the type of x correctly... it always assumes it is of type string.

In the example below, the type of obj outside the for loop is detected correctly. The TypeScript compiler recognizes it as a number. However, the obj inside the for loop is recognized as a string when it should be a number.

let myObj: { [key: number]: number } = { 0: 43, 5: 23 };

let obj = myObj[0]; // let obj: number

for (let obj in myObj) {
    if (obj == 1) {
        // ^ [ts] Operator '==' cannot be applied to types 'string' and 'number'.
        // TypeScript compiler thinks 'obj' is of type 'string' when used in `for in` loop.
    }
}

This casues errors in my code and I'm not sure how to work around this issue. Is it something I'm doing wrong, or is it a TypeScript bug?

What is your use case?

Aluan Haddad
– Aluan Haddad

2017-12-01 07:41:50 +00:00
Commented Dec 1, 2017 at 7:41 — Aluan Haddad
– Aluan Haddad, Commented Dec 1, 2017 at 7:41

jcalz · Accepted Answer · 2017-12-01 02:28:36Z

3

You know that for k in o loops over o's property keys, not its values, right? Additionally, JavaScript only really allows strings as property keys, and will silently coerce non-strings to strings:

From MDN's documentation for accessing properties:

Property names must be strings. This means that non-string objects cannot be used as keys in the object. Any non-string object, including a number, is typecasted into a string via the toString method.

So that means that TypeScript is actually correct when it types obj as string:

for (let obj in myObj) {
    console.log(typeof obj); // "string"
    if (obj === "1") // okay now {
      myObj[obj] = 123; // okay, property value is a number
    }
}

Hope that helps; good luck!

answered Dec 1, 2017 at 2:28

jcalz

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2 Comments

Ari Seyhun Over a year ago

So if I wanted to use the key as a number, I'd be required to use something like parseInt or parseFloat?

jcalz Over a year ago

Yes, something like that (e.g., Number(obj) or +obj)

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