Dot vs brackets
book.key looks for a property named key, which doesn't exist. book.author doesn't exist either: missing properties read as undefined, without an error.
You read and write properties with a dot (obj.name) or square brackets (obj[key]), add them by assigning, remove them with delete, test for them with in or Object.hasOwn, and read deep values safely with optional chaining ?..
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 5 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write getField(obj, field) that returns the value of any field name passed in, or the text "missing" if the object doesn't have that own property. Test it with { a: 1, b: undefined } and the fields a, b and c.
Given const order = { customer: { name: "Asha" } }, log the customer's phone number, or "no phone", in one expression without errors. Then do the same for const order2 = {}.
Start with const settings = { theme: "dark", debug: true }. Remove debug, add fontSize: 16, and log the result.
Explain it without notes
When must you use bracket notation instead of dot notation?
What's the difference between in, Object.hasOwn and comparing with undefined?
What does optional chaining do, and where is it the wrong tool?
Why does obj[{}] usually cause bugs?
Preview
Console
Nothing logged yet.Expected console output
Wings of Fire
180
1999
undefined
undefined