Having trouble iterating over fields of an interface in TypeScript?

I am currently facing an issue while trying to loop through the keys of an object that contains an interface. TypeScript seems to be unable to recognize the type of the key.

interface Resources {
  food?: number
  water?: number
  wood?: number
  coal?: number
  stone?: number
  metal?: number
  oil?: number
  power?: number
}

class Game {
  resources: Resources = {}

  addResources = (newResources: Resources) => {
    Object.keys(newResources).forEach(key => {
      // Error:(17, 11) TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Resources'.
     //  No index signature with a parameter of type 'string' was found on type 'Resources'.
      if (this.resources[key]) {
        this.resources[key] += newResources[key]
      } else {
        this.resources[key] = newResources[key]
      }
    })
  }
}

What is the correct way to define the types so that TypeScript can correctly handle the iteration process?

typescript

Answer №1

Absolutely, the Object.keys() function indeed returns an array of strings intentionally, as discussed in this thread here.

In times of need, using 

(Object.keys(newResources) as Array<keyof Resources>).forEach(...)
can be a lifesaver.

Update:

Another helpful approach is to utilize 

(Object.keys(newResources) as Array<keyof typeof newResources>).forEach(...)
.

Answer №2

In the realm of browsers, the interface is but a mere phantom. TypeScipt solely thrives as a compiler concept. As soon as your code undergoes compilation into JavaScript and begins its execution in the browser environment, the notion of interface dissipates into thin air. The purpose of the interface is to provide strong typing for code during the development phase, only to vanish completely when the code runs.

Answer №3

To eliminate the compiler error, simply include a dynamic property.

interface Items {
  apples?: number
  bananas?: number
  oranges?: number
  grapes?: number
  peaches?: number
  pineapples?: number
  strawberries?: number
  [key: string]?: any
}

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