Function hoisting behavior in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is function hoisting in JavaScript?

Function hoisting means JavaScript moves function declarations to the top of their scope before code runs, so you can call functions before they appear in the code.

intermediate

How does hoisting differ between function declarations and function expressions?

Function declarations are fully hoisted, so you can call them before they appear. Function expressions are not hoisted the same way; if assigned to a variable, only the variable declaration is hoisted, not the assignment.

beginner

What happens if you call a function expression before it is defined?

If you call a function expression before it is assigned, you get a TypeError because the variable is hoisted but undefined at that point.

beginner

Example: What will this code output?
<pre>console.log(greet());
function greet() { return 'Hello!'; }</pre>

It will output 'Hello!' because the function declaration 'greet' is hoisted, so it exists before the console.log runs.

intermediate

Example: What will this code output?

console.log(sayHi());
var sayHi = function() { return 'Hi!'; };

It will cause a TypeError because 'sayHi' is hoisted as a variable but is undefined when called, so you cannot call it as a function yet.

What does function hoisting allow you to do in JavaScript?

A Prevent functions from being overwritten

B Use variables before declaring them

C Call functions before they are declared

D Automatically run functions on page load

Function hoisting moves function declarations to the top, so you can call them before their code appears.

Which of these is fully hoisted in JavaScript?

A Function declaration

B Function expression assigned to var

C Arrow function assigned to let

D Class declaration

Function declarations are fully hoisted with their body; function expressions and arrow functions are not.

What error occurs if you call a function expression before it is assigned?

A ReferenceError

B TypeError

C SyntaxError

D No error

Calling an undefined variable as a function causes a TypeError.

Given this code, what will happen?

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

A 'bar' is printed

B undefined is printed

C ReferenceError: foo is not defined

D TypeError: foo is not a function

Variable 'foo' is hoisted but undefined at the call, so calling it causes a TypeError.

Which statement about function hoisting is true?

A Only function declarations are hoisted, not function expressions

B Both function declarations and expressions are fully hoisted

C Function expressions are hoisted before declarations

D Hoisting only applies to variables

Only function declarations are fully hoisted; function expressions are not.

Explain function hoisting and how it affects calling functions before their definition.

Think about what JavaScript does before running your code.

Describe the difference in hoisting behavior between a function declaration and a function expression assigned to a variable.

Consider what happens to the variable and the function code separately.

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