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

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<string, any[]> = new Map<string, any[]>();

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

Not the OP's direct issue but for users encountering this error for libraries not under their control, one can suppress this error is by adding:

{
  ...
  "suppressImplicitAnyIndexErrors": true,
  ...
}

to the tsconfig.json file.

I used this to get around it so I could use the window object.

//in js code somewhere
window.DataManager = "My Data Manager";


//in strict typescript file
let test = (window as { [key: string]: any })["DataManager"] as string;
console.log(test); //output= My Data Manager