Undefined
intermediateDefinition
A JavaScript value that means "nothing has been put here yet," which you get when code asks for something that was never set.
In the wild
If a customer record has no address, `customer.address` is undefined, and asking for `customer.address.city` crashes with "Cannot read properties of undefined."
More from Programming Basics
Array
An ordered list of items kept together under one name, where each item has a numbered position that usually starts at 0.
async / await
Two JavaScript words that let code wait for a slow step, like loading orders from the internet, before moving on, without freezing the rest of the app. `async` marks a function that will wait, and `await` marks the exact step to wait for (usually a promise, a placeholder for a result that isn't ready yet).
Boolean
A value that can only be true or false, used for yes-or-no questions such as "has this order been paid?"
Callback
A function you hand to another piece of code so it can run it later, when something happens, such as a click, a timer ending, or data arriving.
Data Type
The kind of value a piece of data is, such as text, a number, true or false, or a list, which decides what you can do with it.
Dependency (Package)
A piece of code written by someone else that your project installs and relies on instead of building that part itself.