TypescriptRecordOverload.ts

// Define our record defaults
const fruitDefaults = {
    name: '',
    numberOwned: 0,
    best: false,    
}

// Define our record types with a typescript interface 
interface IFruitParams {
    name: string;
    numberOwned: number;
    best: boolean;
}

// Create our FruitRecord class
export class FruitRecord extends Record(fruitDefaults) {    
    
    // Set the params. This will also typecheck when we instantiate a new FruitRecord
    constructor(params: IFruitParams) {
        super(params);
    }

    // This following line is the magic. It overrides the "get" method of record
    // and lets typescript know the return type based on our IFruitParams interface
    get<T extends keyof IFruitParams>(value: T): IFruitParams[T] { 
        
        // super.get() is mapped to the original get() function on Record
        return super.get(value) 
    }

}

// Now typescript will properly enforce our record types
// and will make sure we don't try to get() any values that aren't declared!

const bestFruit = new FruitRecord({  
  name: 'blueberry',
  numberOwned: 10,
  best: true
})

// Test Case #1
// returns true
isBestFruit(bestFruit.get('name'))

// Test Case #2
// returns typescript error - 'best' (boolean) is not assignable to string
isBestFruit(bestFruit.get('best')) 

// Test Case #3
// returns typescript error - 'worst' property does not exist on bestFruit
const isWorst = bestFruit.get('worst')