Why does VScode sense this string as undefined?

Question

Here's a bare-bones example. I have a class whose objects have two properties:

• a reference to a parent object of the same class
• some value (in our case a string but it doesn't matter what type it is)

Here's the code:

class Foo {
  /**
   * @param {Foo?} parent
   * @param {string} value */
  constructor(parent, value) {
    this.parent = parent;
    this.value = value;
    if(parent) {
      let value = parent.value; // vscode says value is undefined
      console.log(value); // but when printed it's the expected string
    }
  }
}

If I mouse over value on the commented lines, the intellisense popup simply displays let value: undefined, and gives me the red squiggles if I try to do anything with it. Seeing as the program behaves as expected, I assume the issue lies in my JSdoc.

Note: the nullability of parent is not the issue, as this example bears the same problem:

class Foo {
  /**
   * @param {Foo} parent
   * @param {string} value */
  constructor(parent, value) {
    this.parent = parent;
    this.value = value;
    let parentValue = parent.value; // vscode says parentValue is undefined
  }
}

What am I doing wrong? How can I convince intellisense that value is definitely a string?

Answer 1

you should define value in Foo

class Foo {
  value = ''
  /**
   * @param {Foo?} parent
   * @param {string} value */
  constructor(parent, value) {
    this.parent = parent;
    this.value = value;
    if(parent) {
      let value = parent.value; // vscode says value is undefined
      console.log(value); // but when printed it's the expected string
    }
  }
}