What is control flow analysis

TypeScript 2.0+

Type checker analyses all possible flows of control in statements and expressions to produce the most specific type possible (the narrowed type) at any given location for a local variable or parameter that is declared to have a union type.

let test: string | string[];

test = "this is a string";
test.toLowerCase(); // type 'string'

test = ["these", "are", "strings"];
test.join(" "); // type 'string[]'

Above code is in same scope, but the type checker looks for the most specific type possible for the command variable at any given location.

function testFunction(test: string | string[]): string {
  if (typeof test === "string") {
    // executes when test is type of string
    return test.toLowerCase();
  }

  // else, assume it is an array
  return test.join(" ");
}

Same idea, but with null check:

type TOptions = {
  hasToBeString: string;
  nullableString?: string | null | undefined;
};

function testNullable(test: TOptions): string {
  const { hasToBeString, nullableString } = person;

  if (!nullableString) {
    return hasToBeString;
  }
}