how to get value from json via variable in typescript?

Question

supposing I have to read some data from some json files(i18n), every json file may look like:

{
  "foo": "1",
  "bar": "2",
  ...
}

I don't know how many fields this json have(it can be expanded), but it's fields look like

{
 [prop: string]: string
}

besides, all the json files share the same fields. when I try to read a value from this json via:

//a can be expanded, I'm not sure how many fileds does it have
let a = {
    name: "dd",
    addr: "ee",
}

//I'm confident a has a field "name"
let b = "name";

console.log(a[b]);

the error message is:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type

how could I fix it?

I just tested. No any error message. Everything is working properly. — Serge
– Serge, Commented Nov 30, 2021 at 23:59

Terry · Accepted Answer · 2021-11-30 23:55:07Z

1

The error you're encountering is because the keys in a is not just any string (in fact, it can only be "name" or "add"), but b can be a string of any arbitrary value. If you are very sure that b represents a key found in the object a, you can hint TypeScript as such:

let b: keyof typeof a = "name";

Attempting to assign any arbitrary string value to b will lead to an error:

// This will cause an error
let b: key typeof a = "foobar";

See proof-of-concept on TypeScript Playground.

answered Nov 30, 2021 at 23:55

Terry

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