Typescript say variable can be null even if I check before

Question

I have an argument that can be string or null. At the beginning of the function I check to see if this argument is null, if yes, I set a default value.

However, after this check, typescript tells me that this argument can still be null.

Here is an example

pollUrl: function (
    httpClient,
    url: string,
    maxIterations: number | null = null,
    delay = 600
  ): Promise<any> {
    if (maxIterations == null) {
      maxIterations = 25
    }
    return httpClient.get(url).then((result) => {
      return Utils.delay(delay).then(() => Utils.pollUrl(httpClient, url, maxIterations - 1, delay))
    })
  },

in the Utils.pollUrl at the end of the function, typescript is telling me maxIterations: Object is possibly 'null'., even if I check before

Answer 1

What about instead using a default similar to what you have with delay:

pollUrl: function (
    httpClient,
    url: string,
    maxIterations = 25,
    delay = 600
  ): Promise<any> {
    return httpClient.get(url).then((result) => {
      return Utils.delay(delay).then(() => Utils.pollUrl(httpClient, url, maxIterations - 1, delay))
    })
  },

Can use like:

pollUrl(client, 'foo', undefined, 500);