Trying to understand index signatures in TypeScript

Question

I think I basically got how index signatures work in TypeScript. However, there is one thing I don\'t get. Given the following sample code:

const sales: {[ k         


        

Answer 1

This is just an addition to the accepted answer, because it's totally right

If your aim is to get compile time errors if you don't check if e.g. sales.d exists/is not undefined, you could implement your own interface here:

interface SomeDictionary {
  {[ key: string ]: T | undefined }
}

const sales: SomeDictionary = {
  a: 532,
  b: 798,
  c: 264
};

// compile time error
const result = sales.a + sales.b;

// working
if(sales.a !== undefined && sales.b !== undefined){
  const result = sales.a + sales.b;
}

AFAIK there is no such built-in interface in typescript.

I think index signatures (in your case) make sense if you want to iterate over the keys of an such an object:

const sales: {[ key: string ]: number } = {
  a: 532,
  b: 798,
  c: 264
};

let sum = 0;
for(const key in Object.keys(sales)){
  sum = sum + sales[key];
}

I assume there are much more uses cases which are not coming into my mind right now..

To your side question: No, it's not (if you meant Record from here. Record does not even compile, because it needs a second type argument. I would say Record's are not really related to your issue.