TypeScript TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'

Im getting this compilation error in my Angular 2 app:

TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

The piece of code causing it is:

getApplicationCount(state:string) {
    return this.applicationsByState[state] ? this.applicationsByState[state].length : 0;
  }

This however doesn't cause this error:

getApplicationCount(state:string) {
    return this.applicationsByState[state] ? this.applicationsByState[state].length : 0;
  }

This doesn't make any sense to me. I would like to solve it when defining the attributes the first time. At the moment I'm writing:

private applicationsByState: Array = [];

But someone mentioned that the problem is trying to use a string type as index in an array and that I should use a map. But I'm not sure how to do that.

Thans for your help!

If you want a key/value data structure then don't use an array.

You can use a regular object:

private applicationsByState: { [key: string]: any[] } = {};

getApplicationCount(state: string) {
    return this.applicationsByState[state] ? this.applicationsByState[state].length : 0;
}

Or you can use a Map:

private applicationsByState: Map = new Map();

getApplicationCount(state: string) {
    return this.applicationsByState.has(state) ? this.applicationsByState.get(state).length : 0;
}

Stack Overflow Q#40358434, score: 173