Return values in Javascript

Introduction

Return values let a function give back a result after it finishes running. This helps you use that result later in your code.

When you want a function to calculate and give back a number, like adding two numbers.

When you need to get a value from a function to use somewhere else in your program.

When you want to check if something is true or false and use that answer later.

When you want to get a string or message from a function to show to the user.

When you want to stop a function early and send back a value immediately.

Syntax

Javascript

function functionName() {
  // do something
  return value;
}

The return keyword sends a value back to where the function was called.

Once return runs, the function stops running any more code inside it.

Examples

This function adds two numbers and returns the sum.

Javascript

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

This function returns a greeting string.

Javascript

function greet() {
  return "Hello!";
}

This function returns true if the number is even, otherwise false.

Javascript

function isEven(num) {
  return num % 2 === 0;
}

Sample Program

This program defines a function that multiplies two numbers and returns the product. Then it calls the function with 4 and 5, saves the result, and prints it.

Javascript

function multiply(x, y) {
  return x * y;
}

const result = multiply(4, 5);
console.log("The result is", result);

Important Notes

If a function does not have a return statement, it returns undefined by default.

You can return any type of value: numbers, strings, booleans, objects, or even other functions.

Summary

Use return to send a value back from a function.

After return, the function stops running.

Returned values let you use results from functions elsewhere in your code.

Practice

1. What does the return statement do in a JavaScript function?

easy

A. It pauses the function but continues later.

B. It prints a value to the screen.

C. It sends a value back to where the function was called.

D. It creates a new variable inside the function.

Solution

1. Step 1: Understand the purpose of return

   The return statement sends a value back to the place where the function was called.

2. Step 2: Recognize function behavior after return

   After return, the function stops running and no further code inside it executes.

3. Final Answer:

   It sends a value back to where the function was called. -> Option C

4. Quick Check:

   Return sends value back [OK]

Hint: Return sends value back and stops function [OK]

Common Mistakes:

• Thinking return prints output
• Believing return pauses function
• Confusing return with variable creation

2. Which of the following is the correct syntax to return the value 10 from a function?

easy

A. return 10;

B. return = 10;

C. return: 10;

D. return => 10;

Solution

1. Step 1: Identify correct return syntax

   The correct syntax to return a value is return value; with a semicolon.

2. Step 2: Check each option

   return 10; uses return 10; which is correct. return = 10; uses equals sign for assignment, invalid. return: 10; uses colon which is invalid. return => 10; uses arrow syntax incorrectly.

3. Final Answer:

   return 10; -> Option A

4. Quick Check:

   Correct return syntax ends with semicolon [OK]

Hint: Use 'return value;' with semicolon [OK]

Common Mistakes:

• Using colon instead of semicolon
• Confusing arrow functions with return syntax
• Using = for assignment with return

3. What is the output of this code?

function add(a, b) {
  return a + b;
}
console.log(add(3, 4));

medium

A. 7

B. 34

C. undefined

D. NaN

Solution

1. Step 1: Understand function behavior

   The function add takes two numbers and returns their sum using return a + b;.

2. Step 2: Calculate the return value

   Calling add(3, 4) returns 3 + 4 = 7, which is printed by console.log.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Sum of 3 and 4 is 7 [OK]

Hint: Return adds numbers and prints result [OK]

Common Mistakes:

• Concatenating numbers as strings
• Forgetting to return value
• Expecting undefined without return

4. Find the error in this function:

function greet(name) {
  console.log('Hello ' + name);
  return;
  console.log('Goodbye');
}

medium

A. The function syntax is invalid.

B. Missing return value causes error.

C. The function should not use console.log.

D. The function returns before printing 'Goodbye', so 'Goodbye' never shows.

Solution

1. Step 1: Analyze function flow

   The function prints 'Hello ' plus the name, then hits return; which stops the function.

2. Step 2: Check code after return

   Code after return (printing 'Goodbye') never runs, so 'Goodbye' is never printed.

3. Final Answer:

   The function returns before printing 'Goodbye', so 'Goodbye' never shows. -> Option D

4. Quick Check:

   Code after return is skipped [OK]

Hint: Code after return does not run [OK]

Common Mistakes:

• Expecting code after return to run
• Thinking missing return value causes error
• Confusing console.log with return

5. You want a function that returns the first non-empty string from an array or returns 'No valid string' if none found. Which function correctly uses return values?

hard

A. function firstNonEmpty(arr) {
arr.forEach(str => {
if (str) return str;
});
return 'No valid string';
}

B. function firstNonEmpty(arr) {
for (const str of arr) {
if (str) return str;
}
return 'No valid string';
}

C. function firstNonEmpty(arr) {
let result = '';
arr.forEach(str => {
if (str) result = str;
});
return result || 'No valid string';
}

D. function firstNonEmpty(arr) {
for (let i = 0; i < arr.length; i++) {
if (arr[i] === '') return arr[i];
}
return 'No valid string';
}

Solution

1. Step 1: Understand the goal

   The function must return the first non-empty string or a default message if none found.

2. Step 2: Evaluate each function

   function firstNonEmpty(arr) { for (const str of arr) { if (str) return str; } return 'No valid string'; } uses a for-of loop and returns immediately when a non-empty string is found, else returns default. This is correct.
   function firstNonEmpty(arr) { arr.forEach(str => { if (str) return str; }); return 'No valid string'; } uses forEach with return inside callback, which does not stop the outer function.
   function firstNonEmpty(arr) { let result = ''; arr.forEach(str => { if (str) result = str; }); return result || 'No valid string'; } collects last non-empty string but does not return first.
   function firstNonEmpty(arr) { for (let i = 0; i < arr.length; i++) { if (arr[i] === '') return arr[i]; } return 'No valid string'; } returns empty string if found, which is opposite of requirement.

3. Final Answer:

   function firstNonEmpty(arr) { for (const str of arr) { if (str) return str; } return 'No valid string'; } -> Option B

4. Quick Check:

   Return inside loop stops function immediately [OK]

Hint: Use return inside loop to exit early [OK]

Common Mistakes:

• Using forEach with return expecting to exit function
• Returning empty string instead of non-empty
• Returning last instead of first non-empty string