How to fix problem with null ts type checking?

Question

I have property of class that can be null and in code I'm checking if value not null and is array - push new value to it, but type checker still thinks that value can be null. Can somebody explain why and how to fix it, thanks?

class BIT {
  head: BIT | null = this;
  private value: string;
  right: BIT | BIT[] | null = null;
  left: BIT | BIT[] | null = null;

  somefunction(node: BIT) {
    const haveLeft = this.left !== null;
    const leftIsBranch = haveLeft && this.left instanceof Array;

    if (haveLeft) {
      if (leftIsBranch) {
        this.left.push(node);
      }
    }
  }
}

UPD: if I get rid of boolean variables haveLeft and leftIsBranch and explicitly add it to if statement -> everything work just fine. What the heck is going on?

Answer 1

class BIT {
  head: BIT | null = this;
  private value: string;
  right: BIT | BIT[] | null = null;
  left: BIT | BIT[] | null = null; // below you try push: this.left.push(this.value). But your types it's BIT or Array of BIT or null, when 'this.value' is string. 

  somefunction() {
    const haveLeft = this.left !== null;
    const leftIsBranch = haveLeft && this.left instanceof Array;

    if (haveLeft) {
      if (leftIsBranch) {
        this.left.push(value); // you forgot to specify 'this': this.left.push(this.value);
      }
    }
  }
}

Also instead of example: null | BIT example: null | BIT you can specify example?: BIT

Answer 2

In TypeScript all types are nullable by default:

By default null and undefined are subtypes of all other types. That means you can assign null and undefined to something like number.

However, when using the --strictNullChecks flag, null and undefined are only assignable to void and their respective types. This helps avoid many common errors. In cases where you want to pass in either a string or null or undefined, you can use the union type string | null | undefined. Once again, more on union types later on.

[From TS docs ]

So unless you're using the --strictNullChecks compiler flag, adding | null | null is not necessary.

The cause of your type check error is probably that you're checking against null but not undefined - which is the default value of uninitialized fields. A loose equality ( != instead of !== ) check should help identifying also undefined cases:

const haveLeft = this.left != null; // This also excludes `undefined`

Answer 3

Note the following type checking.

 console.log(typeof null); // object console.log(Array.isArray([])); // true