ECMA Script 6 (ES6)

Question 1

What happens when you re-declare a variable defined with let?

Answer 1

You get an error

Question 2

What is one of the more common problems with the var keyword?

Answer 2

You can re-declare variables without receiving an error.

Question 3

What is the difference in scope between variables declared with var and let?

Answer 3

When you declare a variable with the var keyword, it is declared globally, or locally if declared inside a function.

The let keyword behaves similarly, but with some extra features. When you declare a variable with the let keyword inside a block, statement, or expression, its scope is limited to that block, statement, or expression.

Question 4

What’s the difference between declaring variables with let and const?

Answer 4

const variables are read only.

Question 5

What does the const keyword prevent?

Answer 5

Using the const declaration only prevents reassignment of the variable identifier.

Question 6

What type of objects assigned using const are mutable?

Answer 6

Arrays and Objects - it only prevents reassignment.

Question 7

When using a const declaration, how do you prevent your object from being changed?

Answer 7

Object.freeze()

Once the object is frozen, you can no longer add, update, or delete properties from it. Any attempt at changing the object will be rejected without an error.

Question 8

How do you write anonymous functions in ES6?

Answer 8

you can use arrow function syntax:

const myFunc = () => {
  const myVar = "value";
  return myVar;
}

Question 9

How do you write an anonymous function with no function body in ES6?

Answer 9

const myFunc = () => “value”;

Question 10

What’s the syntax for default parameters?

Answer 10

const greeting = (name = “Anonymous”) => “Hello “ + name;

Question 11

How do you pass a variable number of arguments to function?

Answer 11

ES6 introduces the rest parameter for function parameters. With the rest parameter, you can create functions that take a variable number of arguments. These arguments are stored in an array that can be accessed later from inside the function.

Check out this code:

function howMany(...args) {
  return "You have passed " + args.length + " arguments.";
}