You can get the length of an array in JavaScript using

arrayVariable.length

Roughly like this:

arrayVariable = [1, 2, 3, 4, 5]
console.log(arrayVariable.length);

--

In your code you have not created an array. allCars is an object.

You could get the number of properties in your custom object using Object.keys():

var numberOfKeys = Object.keys(allCars).length;

If you need this, I have concerns about the underlying data model, though.

--

You could also use a map data structure instead of an object and then use the size property.

let allCars = new Map();
allCars.set('Ford, 'Ka);
allCars.set('GM', 'Sonic');
allCars.set('Audi', 'A4');
console.log(allCars.size);

But, I think this is going down a very different route than the code you already shared.

Just do something like:

const _stepData = (typeof stepData === 'string') ? [stepData] : stepData;

then iterate it as an Array. More consistent, less bugs.

This might help with your first case: https://stackoverflow.com/a/52490977/12414867

For example:

type TupleEq<TItem, TLength extends number> = [TItem, ...TItem[]] & { length: TLength };
const a: TupleEq<number, 4> = [1, 2, 3, 4];

As for the second case, I'm not sure if it's possible in TS :/

I'm guessing you need to wait for the Promise to resolve first, then you can access the response and count the objects inside the array.

this.swimbandsService.getZones().then((response) => {
  // might need to parse the response as json, depending on your service logic
  var lengthZones = response.length;
});