How to use null check in TypeScript in function instead of if?

Question

typescript

It is easier to explain the question with a simple example

const isNotEmptyString = (str: string|null): boolean => {
  if (!str) {
    return false;
  }
  return !!str.length;
};

const getStringLength = (str: string|null): number => {
  if (!isNotEmptyString(str)) {
    return 0;
  }
  return str.length; // Qualifier of 'length' is possibly null
};

In "isNotEmptyString()" I check the parameter for "null" - if it is "null", the function will return ``false``.
In "getStringLength()" I call "isNotEmptyString()" and if "isNotEmptyString()" returns true, then it is logical that the passed parameter "str" ​​will not be "null". But TypeScript still requires a check for "null" in the "getStringLength()" method.
Is there any way to avoid the extra "if (str)" in "getStringLength()"?

Answer 1

can. it's called type-predicate

const isNotEmptyString = (str: string|null): str is string => { ...