give type to accumulator with array reduce typescript

Question

I am generating object with time intervals with some func, it returns ["00:00","00:30","01:00"] ... , but for my purpose i need to have obj-map {{"00:00": "00:00"}, {"00:30":"00:30"}, {"01:00":"01:00"}} I have trouble typing the reduce func below, how can i type acc or the return value of the func to avoid using any as type for acc ?

generateTimeIntervals(0, 1440, 30).reduce((acc, val) => {
  acc[val] = val;
  return acc;
}, {})

Answer 1

how can i type acc or the return value of the func to avoid using any as type for acc?

You'd do that by providing the type of the {} seed you're providing, for instance Record<string, string> (but the exact type isn't the point here, the point is that assigning a type is how you do what you asked):

let result = generateTimeIntervals(0, 1440, 30).reduce((acc, val) => {
  acc[val] = val;
  return acc;
}, {} as Record<string, string>)

TypeScript will be able to follow the reduce call's logic sufficiently to type the return value for you based on that.

---

It's not entirely clear to me from the question whether your reduce code also needs some tweaking, but that's a different question...

Answer 2

You can pass in the type as an argument to reduce :

let result = generateTimeIntervals(0, 1440, 30).reduce<Record<string, string>>((acc, val) => {
  acc[val] = val;
  return acc;
}, {})

See an example here: https://www.typescriptlang.org/play?#code/GYVwdgxgLglg9mABAcwKZlQJwIZVQFRgFtUBJMPTAN2wBsBnACmwC4wQiAjLAGkU5aJ2XXogiDh3TAEpEAbwCwAKESJMqKCExIA2stWqAylEwwwyZtL7HT5xpyuIbZixGn7EAXWUBfZctoNNVR6EFooRABeFHQsXAJiMgosGgZGAAY+AEYAFhzMxABmdOkAOnUAExAIVAAeACVUCDhMCtr6Exc+DttkAD4+xmYICD5U2Ui++Q9sEZ1UzyjEVIBuD3VNbURZiDWlHz45H3clZWawejhA0to4CwAidVDw+74nsKhpIA