Exploring the functionality of this TypeScript code: What's the distinction between { [key: string]: string }[] and { prop1: string, prop2: string }[]

Below is the code I am currently working with:

  get tags(): { [key: string]: string }[] {
    let tags: { [key: string]: string }[] = [];

    if(this.tags) {
      Object.keys(this.tags).forEach(x => {
        tags.push({ prop1: this.tags[x], prop2: getVal(x)});
      });
    }
    return tags;
  }

I find it strange that the return type matches even though it seems like it shouldn't. Can someone clarify why this is happening?

Would it be better to use the following code instead?

      get tags(): { prop1: string, prop2: string }[] {
        let tags: { prop1: string, prop2: string }[] = [];
    
        if(this.tags) {
          Object.keys(this.tags).forEach(x => {
            tags.push({ prop1: this.tags[x], prop2: getVal(x)});
          });
        }
        return tags;
      }

Answer №1

Index Signatures, as defined on Typescript's documentation, are a useful tool.

{ [key: string]: string }[]

They come in handy when the keys of an object type are not all known beforehand. By specifying that the key will be of type string, Typescript allows for this flexibility.

This approach eliminates any complaints from Typescript in the initial snippet.

The second question may vary depending on personal opinion. However, if your code only ever involves two keys like prop1 and prop2, it might be beneficial to enforce stricter checks to prevent additional keys like prop3 being added by other developers.

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