TypeScript · Senior · Question 62 of 75

What does a `this is` predicate do in a class method?

class Node {
  isLeaf(): this is Leaf { return this.children.length === 0; }
}

✓Correct answer: It narrows the object the method was called on when the method returns true
2It changes this at runtime
3It makes the method static

Explanation

Calling if (node.isLeaf()) narrows node inside the branch, like a regular type guard.

Further reading

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