Function hoisting behavior in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code with function declarations and calls?

Consider the following JavaScript code. What will be printed to the console when it runs?

Javascript

console.log(foo());
function foo() {
  return 'Hello from foo';
}

A ReferenceError: foo is not defined

B "Hello from foo"

C TypeError: foo is not a function

D undefined

💡 Hint

Function declarations are hoisted completely, so they can be called before their definition.

In JavaScript, function declarations are hoisted with their body, so calling foo() before its declaration works fine and returns the string.

❓ Predict Output

intermediate

What happens when calling a function expression before its definition?

Look at this code snippet. What will be the result when it runs?

Javascript

console.log(bar());
var bar = function() {
  return 'Hello from bar';
};

A TypeError: bar is not a function

B "Hello from bar"

C ReferenceError: bar is not defined

D undefined

💡 Hint

Variable declarations are hoisted but not their assignments.

The variable bar is hoisted but initialized as undefined at the time of the call, so calling bar() causes a TypeError because undefined is not a function.

❓ Predict Output

advanced

What is the output when a function declaration and variable share the same name?

Analyze this code and determine what it prints to the console.

Javascript

console.log(baz);
var baz = 5;
function baz() {
  return 10;
}
console.log(typeof baz);

A

5
number

B

function baz() { return 10; }
function

C

function baz() { return 10; }
number

D

undefined
number

💡 Hint

Function declarations are hoisted before variable declarations, but variable assignments override the function.

The function baz is hoisted first, so the first console.log prints the function representation. The var baz = 5 then performs the assignment, overriding baz with 5. Thus, typeof baz is 'number'.

❓ Predict Output

advanced

What error occurs when calling a let-declared function before its definition?

What happens when this code runs?

Javascript

console.log(qux());
let qux = function() {
  return 'Hello from qux';
};

A ReferenceError: Cannot access 'qux' before initialization

B TypeError: qux is not a function

C "Hello from qux"

D undefined

💡 Hint

Variables declared with let are not hoisted like var and cannot be accessed before initialization.

Using let means the variable is in a temporal dead zone until initialized, so accessing qux before assignment throws a ReferenceError.

🧠 Conceptual

expert

Which option correctly explains function hoisting behavior in JavaScript?

Select the statement that best describes how function declarations and function expressions are hoisted in JavaScript.

A Function expressions are hoisted with their full definitions, but function declarations are hoisted only as variable declarations.

B Function declarations and function expressions are both hoisted with their full definitions, so they can be called anywhere in the code.

C Neither function declarations nor function expressions are hoisted; they must be defined before use.

D Function declarations are hoisted with their full definitions, but function expressions assigned to variables are hoisted only as variable declarations without their assignments.

💡 Hint

Think about how JavaScript treats function declarations differently from variables assigned functions.

Function declarations are hoisted completely, so they can be called before their definition. Function expressions assigned to variables are hoisted only as variable declarations, so the variable is undefined before assignment.

Practice

1. Which statement best describes function hoisting in JavaScript?

easy

A. Function declarations are hoisted and can be called before they are defined.

B. Function expressions are hoisted and can be called before assignment.

C. Variables declared with let are hoisted like functions.

D. Arrow functions are hoisted like function declarations.

Solution

1. Step 1: Understand hoisting for function declarations

   Function declarations are moved to the top of their scope, so they can be called before their definition in code.

2. Step 2: Compare with function expressions and other types

   Function expressions and arrow functions are not hoisted in the same way; they cannot be called before assignment.

3. Final Answer:

   Function declarations are hoisted and can be called before they are defined. -> Option A

4. Quick Check:

   Function declarations hoisted = C [OK]

Hint: Only function declarations are hoisted fully [OK]

Common Mistakes:

• Thinking function expressions are hoisted like declarations
• Confusing variable hoisting with function hoisting
• Assuming arrow functions are hoisted

2. Which of the following is the correct syntax to declare a function that will be hoisted?

easy

A. function myFunc() { return 1; }

B. var myFunc = () => 1;

C. let myFunc = () => 1;

D. const myFunc = function() { return 1; };

Solution

1. Step 1: Identify function declaration syntax

   Function declarations use the function keyword followed by the function name and body.

2. Step 2: Check other options for hoisting behavior

   Options A, B, and C are function expressions or arrow functions assigned to variables, which are not hoisted as functions.

3. Final Answer:

   function myFunc() { return 1; } -> Option A

4. Quick Check:

   Function declaration syntax = D [OK]

Hint: Only function declarations use 'function name()' syntax [OK]

Common Mistakes:

• Confusing function expressions with declarations
• Assuming arrow functions are hoisted
• Using variable assignment syntax for hoisted functions

3. What will be the output of the following code?

console.log(foo());

function foo() {
  return 'Hello';
}

console.log(bar());

var bar = function() {
  return 'World';
};

medium

A. SyntaxError

B. ReferenceError for bar

C. 'Hello' followed by 'World'

D. 'Hello' followed by TypeError

Solution

1. Step 1: Analyze the call to foo()

   The function foo is a declaration and hoisted, so foo() returns 'Hello'.

2. Step 2: Analyze the call to bar()

   bar is a variable assigned a function expression. The variable is hoisted but initialized as undefined at the time of the call, so calling bar() causes a TypeError.

3. Final Answer:

   'Hello' followed by TypeError -> Option D

4. Quick Check:

   Function declaration hoisted, function expression not initialized = A [OK]

Hint: Function declarations hoisted, expressions not initialized [OK]

Common Mistakes:

• Expecting both functions to run without error
• Confusing ReferenceError with TypeError
• Assuming variable functions are hoisted fully

4. Consider the code below. It throws an error. What is the fix?

console.log(add(2, 3));

const add = function(a, b) {
  return a + b;
};

medium

A. Move the console.log after the function expression.

B. Use var add instead of const add.

C. Change const add to function add declaration.

D. Wrap the function expression in parentheses.

Solution

1. Step 1: Identify why error occurs

   The variable add is declared with const and assigned a function expression. It is not hoisted as a function, so calling it before assignment causes a ReferenceError.

2. Step 2: Fix by using function declaration

   Changing to a function declaration function add(a, b) hoists the function, allowing calls before its definition.

3. Final Answer:

   Change const add to function add declaration. -> Option C

4. Quick Check:

   Function declarations hoisted, expressions not [OK]

Hint: Use function declarations to call before definition [OK]

Common Mistakes:

• Assuming var fixes hoisting for function expressions
• Thinking moving console.log is the only fix
• Trying to wrap function expression in parentheses

5. You want to organize your code so that helper functions can be called anywhere in the file without errors. Which approach best uses function hoisting to achieve this?

hard

A. Assign all helper functions to const variables at the top of the file.

B. Use function declarations anywhere; they are hoisted to the top automatically.

C. Declare all helper functions using function declarations at the bottom of the file.

D. Wrap helper functions in arrow functions assigned to let variables.

Solution

1. Step 1: Understand hoisting behavior for function declarations

   Function declarations are hoisted to the top of their scope, so they can be called anywhere in the file regardless of their position.

2. Step 2: Compare with other approaches

   Assigning functions to variables (const or let) does not hoist the function body, so calls before assignment cause errors.

3. Final Answer:

   Use function declarations anywhere; they are hoisted to the top automatically. -> Option B

4. Quick Check:

   Function declarations hoisted anywhere = A [OK]

Hint: Function declarations can be called anywhere due to hoisting [OK]

Common Mistakes:

• Thinking function expressions are hoisted
• Placing function declarations only at bottom
• Using arrow functions expecting hoisting