ES6+ Quiz - Languages JavaScript

Free practice quiz: ES6+ (Languages JavaScript). 56 multiple-choice questions with answers and explanations. Play online, no sign-up needed.

Sample questions

1. Which of the following correctly declares a constant variable in JavaScript ES6?

   1. var PI = 3.14;
   2. let PI = 3.14;
   3. const PI = 3.14;
   4. constant PI = 3.14;
   Show answer

   Answer: C. const PI = 3.14;

   In ES6, the `const` keyword is used to declare variables whose values cannot be reassigned. `var` and `let` declare variables that can be reassigned, and `constant` is not a valid JavaScript keyword for variable declaration.

2. What does the `async` keyword in ES6 signify?

   1. It declares a function that will always return a string
   2. It declares a function that will automatically pause execution until a Promise resolves
   3. It declares a function that implicitly returns a Promise
   4. It declares a function that is asynchronous by nature
   Show answer

   Answer: C. It declares a function that implicitly returns a Promise

   An `async` function always returns a Promise. If the function returns a non-Promise value, it's wrapped in a resolved Promise. If it throws an error, it returns a rejected Promise.

3. Consider the code: `let x = 10; function outer() { let x = 20; function inner() { console.log(x); } inner(); } outer();`. What is logged?

   1. 10
   2. 20
   3. undefined
   4. Error
   Show answer

   Answer: B. 20

   JavaScript uses lexical scoping. The `inner` function is within the scope of `outer`, which has its own `x` variable with the value `20`. Therefore, `inner` accesses `x` from `outer`'s scope, not the global scope.

4. What is the purpose of the `Symbol.iterator` method in ES6?

   1. To define how an object can be iterated over
   2. To create a new symbol
   3. To perform mathematical operations
   4. To handle asynchronous operations
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   Answer: A. To define how an object can be iterated over

   The `Symbol.iterator` is a method that is called when a `for...of` loop or other iterable consumption methods are used on an object. It returns an iterator object, which implements the iterator protocol.

5. What is a WeakMap in ES6?

   1. A Map where all keys must be weak references
   2. A Map where keys are weakly held, allowing garbage collection if the key is no longer referenced elsewhere
   3. A Map that only stores weak references
   4. A Map that throws an error if a key is too large
   Show answer

   Answer: B. A Map where keys are weakly held, allowing garbage collection if the key is no longer referenced elsewhere

   Unlike a `Map`, keys in a `WeakMap` must be objects and are held weakly. If an object used as a key is garbage collected, the entry in the `WeakMap` is automatically removed.

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