Typescript assign value for keyof not working

Question

I have a problem:
- I do not assign for value of variable temp as code below
Please, how to assign value to variable temp
Following code below:

interface Base {
      a: number;
      b: number;
      c: string;
 }

interface Child extends Base {
  d: string;
}

const data: Child = {
  a: 1,
  b: 2,
  c: "1",
  d: "2"
}

function check<T extends Base>(obj: T, key: string) {
  const keys: any[] = Object.keys(obj);
  type dataTypes = keyof T;
  for (let m in keys) {

    // error: 
    // Type 'string' is not assignable to type 'keyof T'.
    //  Type 'string' is not assignable to type '"a" | "b" | "c"
    const temp: dataTypes = m; //error

  }
}

function test(data: Base) {
  check(data, "d")
}

keys is of type string[] not dataTypes[]

Mohamed Farouk
– Mohamed Farouk

2020-01-08 06:04:12 +00:00
Commented Jan 8, 2020 at 6:04 — Mohamed Farouk
– Mohamed Farouk, Commented Jan 8, 2020 at 6:04

Kirill Simonov · Accepted Answer · 2020-01-08 06:26:52Z

2

Object.keys(obj) returns string[], that's why m is a string, but dataTypes is "a" | "b" | "c" (union type), i.e more restrictive. So you can't assign a string to a variable of type "a" | "b" | "c".

The solution is simple: don't use Object.keys(obj):

function check<T extends Base>(obj: T, key: string) {
  type dataTypes = keyof T;
  for (let m in obj) {
    const temp: dataTypes = m;
  }
}

answered Jan 8, 2020 at 6:26

Kirill Simonov

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Comments

Mohamed Farouk · Accepted Answer · 2020-01-08 06:11:18Z

1

Replace for (let m in keys) { by for (var i=0; i< keys.length; i++) {

for (var i=0; i< keys.length; i++) {

    const temp: dataTypes = keys[i]; //no error

  }

answered Jan 8, 2020 at 6:11

Mohamed Farouk

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Comments

cobrexus · Accepted Answer · 2020-11-12 09:28:50Z

-1

Use:

const temp = m as keyof T;

edited Nov 12, 2020 at 9:28

cobrexus

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answered Oct 17, 2020 at 14:43

user14468416

1

2 Comments

Rosmarine Popcorn Over a year ago

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

Jeremy Caney Over a year ago

@Burimi: This looks like an answer to me. It would definitely benefit from more explanation, and it's not entirely clear to me how it differs from similar answers with slightly different syntax, but it's nevertheless an attempt to answer.

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