JavaScript — Test 1

Correct Answer: C. true and false
Explanation: Loose equality treats null and undefined as equal to each other only. Strict equality also compares types. ALSO KNOW (related points): • What does [] + {} evaluate to in JavaScript? → "[object Object]" • What does "b" + "a" + +"a" + "a" evaluate to? → "baNaNa"

Correct Answer: B. [1, 10, 2]
Explanation: With no comparator, sort compares elements as strings. Use .sort((a, b) => a - b) for numbers. ALSO KNOW (related points): • With let arr = [1, 2, 3]; arr[10] = 5;, what is arr.length? → 11 • What does document.querySelectorAll('.temp') return? → A static NodeList of all the matching elements

Correct Answer: C. undefined
Explanation: var declarations are hoisted and initialized to undefined. With let or const, accessing x first throws a ReferenceError (the temporal dead zone). ALSO KNOW (related points): • In which phase do event listeners run by default with addEventListener? → The bubbling phase • What is 'event delegation'? → Attaching one listener to a parent element and handling events from its children through bubbling, using event.target

Correct Answer: B. 3 3 3
Explanation: var is function-scoped, so all the callbacks share one i, which is 3 by the time they run. let would give 0 1 2. ALSO KNOW (related points): • What does localStorage.setItem('n', 5); typeof localStorage.getItem('n') give? → "string"

Correct Answer: A. A D C B
Explanation: Synchronous code runs first, then microtasks (promise callbacks), then macrotasks (setTimeout). ALSO KNOW (related points): • What is a closure in JavaScript? → A function bundled with references to the variables of the scope where it was created, even after that scope has finished • Which statement about Node.js is correct? → It is a JavaScript runtime built on V8 that uses an event-driven, non-blocking I/O model, mainly for server-side code

Correct Answer: B. It works; const prevents reassigning the variable, not mutating the object
Explanation: Use Object.freeze to prevent changes to an object's properties.

Correct Answer: C. 5
Explanation: The number is first converted to the string "5e-7", and parseInt then stops at the 'e'. ALSO KNOW (related points): • What does Math.max() (called with no arguments) return? → -Infinity • What does Array.prototype.map() return? → A new array with the callback's result for each element; the original array is unchanged

Correct Answer: A. "[object Object]"
Explanation: Both operands are converted to strings: [] becomes "" and {} becomes "[object Object]".

Correct Answer: A. 11
Explanation: length is always the highest index plus 1, and the gap in between is filled with empty slots.

Correct Answer: D. -Infinity
Explanation: -Infinity is the identity value for max: any number is greater than it.