Index signature of type 'string' on interface not recognized (typescript)

Question

I have this code:

const sectionInstance: FieldValues = sectionInstances[i]
for (const field in sectionInstance) {
    console.log(sectionInstance[field])
}

field here is of course a string. Here is the type definition for FieldValues:

export interface FieldValues = {
  [key: string]: FieldValue;
}

Still I get this error:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'FieldValues'. No index signature with a parameter of type 'string' was found on type 'FieldValues'.Vetur(7053)

Haven't I declared an index signature of type 'string' for interface FieldValues? Why do I get this error?

Alexander Alexandrov · Accepted Answer · 2020-08-02 17:49:25Z

2

Just remove = from interface definition.

Please take a look at playground

interface FieldValues {
  [key: string]: FieldValue;
}
...
const sectionInstance: FieldValues = sectionInstances[i];
for (const field in sectionInstance) {
  console.log(sectionInstance[field]);
}

edited Aug 2, 2020 at 17:49

answered Aug 2, 2020 at 17:05

Alexander Alexandrov

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5 Comments

Alexander Alexandrov Over a year ago

Also, according to generic coding rules, it is not a good idea to have plural names of types

Bergi Over a year ago

"Please take a look at playground" - why? What's there? What does the linked playground code demonstrate? Please state that in your answer.

Alexander Alexandrov Over a year ago

Just remove = from interface definition:

Mattias Over a year ago

What's the alternative to have plural name of this type?

Alexander Alexandrov Over a year ago

For example, FieldValueMap.It mostly about distance between plural and singular form. I mean FieldValue vs FieldValues and FieldValue vs FieldValueMap

Kavian Rabbani · Accepted Answer · 2020-08-02 17:06:12Z

0

As I know, Typescript can not infer the key type from [key: string]. it's only useful when you are trying to generalize that some keys(probably except the others) might be there with string type. TS cannot ensure that the key you are accessing in the for is of the type string you used in [key: string]. you should say it explicitly.

answered Aug 2, 2020 at 17:06

Kavian Rabbani

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