TypeScript error: The element implicitly assumes an 'any' type due to the inability to index type 'TestObject' with an expression of type 'string'

... There doesn't seem to be an index signature with a parameter of type 'string' on the 'TestObject' type.
I am attempting to iterate through an object in TypeScript using the code below.
Interestingly, it works in StackBlitz but not in my Angular project within VS Code...
A similar or possibly the same question has been discussed here, where they recommend either creating a new type or resorting to 'any'.
Given that it runs successfully in StackBlitz but not in VS Code, I suspect there might be an issue with my project settings, although I'm unsure where to start looking.

export class HelloComponent {
  @Input() name: string;
  myTestObject: TestObject = {
    id: 1,
    name: 'Bob',
    age: 40,
    gender: 'male',
  };
  constructor() {
    this.iterateThroughObject();
  }
  iterateThroughObject(): void {
    Object.keys(this.myTestObject).forEach((property) => {
      console.log(this.myTestObject[property]);
    });
  }
}

The 'TestObject' type is defined in another class

export interface TestObject {
  id: number;
  name: string;
  age: number;
  gender: string;
}

Is this possibly an issue with my tsconfig.json file?

/* To learn more about this file see: https://angular.io/config/tsconfig. */
{
  "compileOnSave": false,
  "compilerOptions": {
    "baseUrl": "./",
    "outDir": "./dist/out-tsc",
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "sourceMap": true,
    "declaration": false,
    "downlevelIteration": true,
    "experimentalDecorators": true,
    "moduleResolution": "node",
    "importHelpers": true,
    "target": "es2020",
    "module": "es2020",
    "lib": [
      "es2020",
      "dom"
    ]
  },
  "angularCompilerOptions": {
    "enableI18nLegacyMessageIdFormat": false,
    "strictInjectionParameters": true,
    "strictInputAccessModifiers": true,
    "strictTemplates": true
  }
}

Answer №1

It appears that the issue at hand is not related to a tsconfig problem. The root cause lies in the fact that when calling Object.keys(this.myTestObject), it returns a string[]. Your definition of TestObject only considers properties such as "id", "name", "age", and "gender" as valid for indexing, rather than any generic string.

In essence, Object.keys loses specific key information of the object it operates on (in this scenario, myTestObject). As a result, it defaults to a more general type string, which cannot be utilized for indexing myTestObject.

There are various solutions to address this challenge. One straightforward approach involves defining a type like

type MyTestObjectKey = "id" | "name" | "age" | "gender"
and then performing an explicit cast:

(Object.keys(myTestObject) as MyTestObjectKey[]).forEach((key) => ...)

To enhance safety measures, you could also explicitly declare:

const MyTestObjectKeyArray: MyTestObjectKey[] = ["id", "name", "age", "gender"];
MyTestObjectKeyArray.forEach((key) => ...)

I trust this guidance proves beneficial!

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