JavaScript Flashcards

What is the output of `console.log([] + [])`?

Answer: ""

Both arrays are coerced to empty strings, and string concatenation produces an empty string `""`.

Which of the following correctly uses destructuring to extract values from an object?

Answer: const {name, age} = person;

Object destructuring uses curly braces to extract named properties into variables.

What does `event.stopPropagation()` do in JavaScript?

Answer: Stops the event from bubbling up to parent elements

`stopPropagation()` halts the event from traveling up (or down) the DOM tree after the current handler executes.

What is a JavaScript generator function?

Answer: A function declared with `function*` that can pause and resume execution using `yield`

Generator functions use `function*` syntax and `yield` to produce a sequence of values on demand, pausing execution between yields.

What does `localStorage.setItem('key', value)` do when `value` is an object?

Answer: Stores `[object Object]` as a string

`localStorage` only stores strings, so objects are coerced via `.toString()`, producing `[object Object]` unless you manually call `JSON.stringify()`.

Which method is used to attach an event handler to a DOM element in modern JavaScript?

Answer: element.addEventListener()

`addEventListener()` is the standard method to attach event handlers, allowing multiple handlers on the same element.

What is the purpose of the `WeakMap` in JavaScript?

Answer: A Map whose keys must be objects and are held weakly so they can be garbage collected

`WeakMap` keys must be objects and are weakly referenced, meaning they don't prevent garbage collection when no other references exist.