By the end of this lesson
Read a record and validate its own keys.
Before you start
- Node.js 24 with a terminal and plain-text editor; the browser-only lesson is labelled separately.
- Complete lesson 8 or be comfortable with its examples.
Three words to know
- Property
- A named value associated with an object.
- Own property
- A property stored directly on an object.
- Prototype
- Another object used for inherited property lookup.
1. A record has named fields
Objects are useful when a value belongs to a named field. Dot notation uses a fixed property name; brackets can use a name stored in a variable. Property access does not validate a record.
const task = { title: "Read", status: "open" };
const field = "status";
console.log(task.title);
console.log(task[field]);
What you should see · example output
Read open
2. Missing and present are different
Object.hasOwn checks a property stored directly on the object. Reading a missing property returns undefined, but a property can also explicitly contain undefined. The in operator includes inherited properties, which is not the rule we want for validating JSON fields.
const record = { status: undefined };
console.log(Object.hasOwn(record, "status"));
console.log(Object.hasOwn(record, "title"));
console.log(Object.hasOwn(record, "toString"));
What you should see · example output
true false false
3. Validate before treating external data as a record
typeof null and typeof [] both return object. Reject null and arrays when the format expects an ordinary record, then check Object.keys and the required value types. Do not merge unknown keys into configuration objects merely because JSON parsed successfully. Dynamic dictionaries are often clearer as a Map.
const record = { title: "Read", status: "open" };
console.log(Object.keys(record).sort().join(","));
What you should see · example output
status,title
YOUR TURN
A small check before you continue
Why is typeof value === "object" not enough to validate a task record?
Show a suggested answer
It includes null and arrays and says nothing about required keys or their types. Validate each of those conditions explicitly.
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.