Implementing a really simple Elvis Operator in TypeScript

Here is a simple routine that implements what is known as the Elvis Operator in TypeScript.

In C# you can write code like this

Nullable<int> age = person?.BestFriend?.Mother?.CurrentHusband?.Age);

If any of the values along the way it will return a null rather than throwing a NullReferenceException.

Using the class in my previous blog Get a Lambda expression as a string it is possible to achieve the same in TypeScript using the following format

const age = elvisOperator(person, x => x.bestFriend.mother.currentHusband.age;

It doesn’t currently support anything more than simple property access, so you can’t access elements in an array for example.

import { Expression } from './expression';

export function isUndefinedOrNull(value: any): boolean {
  return _.isNull(value) || _.isUndefined(value);
}

export function elvisOperator(instance: TSource, member: (v: TSource) => TResult): any {
  let path = Expression.pathAsArray(member);
  let result = instance;
  do {
    if (!isUndefinedOrNull(result) && path.length > 0) {
      result = result[path[0]];
      path = path.slice(1);
    }
  } while (path.length > 0 && !isUndefinedOrNull(result));
  return result;
}