For types like those which are outside your control you can use type guards[1]:

    if (value == null) {
        // TS knows value is null, doesn't let you access value .length for example
    } else if (Array.isArray(value)) {
        // TS knows value is an array
    } else {
        // TS knows that it's an object here
    }

However, having types like those in your own code (e.g. things like X[] | Y | null) is probably an antipattern and should be avoided. Using discriminated unions[2] is much better.

[1]: https://www.typescriptlang.org/docs/handbook/advanced-types.... ("Type Guards and Differentiating Types" section) [2]: [1]: https://www.typescriptlang.org/docs/handbook/advanced-types.... ("Discriminated Unions" section)