Typescript how to fix this in a recursive function inside a function

Question

I am trying to convert a JS function i found to typescript but i am unable to handle correct the this argument:

 getProperty(object: {}, key: string) {
    function iter(a: string) {
      // @ts-ignore
      const item = this ? this[a] : a;
      // @ts-ignore
      if (this && a === key) {
        return result.push(item);
      }
      if (Array.isArray(item)) {
        return item.forEach(iter);
      }
      if (item !== null && typeof item === 'object') {
        return Object.keys(item).forEach(iter, item);
      }
    }
    const result: string[] = [];
    Object.keys(object).forEach(iter, object);
    return result;
  }

I tried the bind and other suggestions that i found in the web but the problem is that the function stops working. To make it working i keep the ts ignore lines. I first tried to convert the iter function into arrow but stops working properly and then the problem remains in the code * this[a]*. Any suggestions to fix this? Thank you

Answer 1

What you are searching for is this: https://www.typescriptlang.org/docs/handbook/2/functions.html#declaring-this-in-a-function

You can add a this with a type to the iter function.

For your example this could look like:

function getProperty(object: {}, key: string) {
    function iter(this: Record<string, any>, a: string) {
        const item = this ? this[a] : a;
        if (this && a === key) {
            return result.push(item);
        }
        if (Array.isArray(item)) {
            return item.forEach(iter);
        }
        if (item !== null && typeof item === 'object') {
            return Object.keys(item).forEach(iter, item);
        }
    }
    const result: string[] = [];
    Object.keys(object).forEach(iter, object);
    return result;
}