Dynamic typing in JavaScript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code with dynamic typing?

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

Javascript

let x = '5';
x = x + 3;
console.log(x);

A undefined

B 8

C TypeError

D "53"

💡 Hint

Remember that + operator with a string converts the other operand to string.

In JavaScript, when you add a number to a string, the number is converted to a string and concatenated. So '5' + 3 becomes '53'.

❓ Predict Output

intermediate

What is the type of variable after reassignment?

What will be the output of the following code?

Javascript

let data = 10;
data = 'hello';
console.log(typeof data);

A "number"

B "undefined"

C "string"

D "object"

💡 Hint

Variables in JavaScript can hold any type and can be reassigned.

Initially data is a number, but after reassignment it holds a string, so typeof returns 'string'.

❓ Predict Output

advanced

What is the output of this dynamic type coercion?

What will this code print to the console?

Javascript

console.log('5' - 3);

A 2

B "53"

C NaN

D TypeError

💡 Hint

The - operator converts strings to numbers if possible.

The - operator triggers numeric conversion, so '5' becomes 5 and 5 - 3 equals 2.

❓ Predict Output

advanced

What error does this code raise due to dynamic typing?

What happens when this code runs?

Javascript

let a = null;
console.log(a.toUpperCase());

A TypeError

B null

C undefined

D SyntaxError

💡 Hint

null has no methods and cannot be used like a string.

Calling a method on null causes a TypeError because null is not an object.

🧠 Conceptual

expert

Which option shows the correct dynamic type change and output?

Which code snippet will output the number 10 after dynamic typing changes?

A let val = '5'; val = val - '5'; console.log(val);

B let val = '5'; val = +val + 5; console.log(val);

C let val = 5; val = val + '5'; console.log(val);

D let val = '5'; val = val + 5; console.log(val);

💡 Hint

The unary + converts string to number before addition.

Option B converts '5' to number 5 using unary +, then adds 5 to get 10. Others produce strings or 0.

Practice

1. What does dynamic typing in JavaScript mean?

easy

A. Variables must be declared with a fixed type

B. Variables can hold any type and change types anytime

C. JavaScript does not allow variables

D. Types are checked only at compile time

Solution

1. Step 1: Understand variable typing in JavaScript

   JavaScript variables do not have fixed types and can hold any value type.

2. Step 2: Define dynamic typing

   Dynamic typing means variables can change their type during program execution.

3. Final Answer:

   Variables can hold any type and change types anytime -> Option B

4. Quick Check:

   Dynamic typing = variables change types [OK]

Hint: Remember: JavaScript variables are flexible with types [OK]

Common Mistakes:

• Thinking variables have fixed types like in Java
• Confusing dynamic typing with static typing
• Believing JavaScript checks types before running

2. Which of the following is the correct way to declare a variable that can hold any type in JavaScript?

easy

A. let x = 5;

B. int x = 5;

C. var x: number = 5;

D. const x = 'hello';

Solution

1. Step 1: Identify JavaScript variable declaration syntax

   JavaScript uses let, var, or const without type annotations.

2. Step 2: Check each option

   let x = 5; uses let correctly. int x = 5; uses int which is not valid in JavaScript. var x: number = 5; uses TypeScript syntax. const x = 'hello'; is valid but declares a constant, not a variable that can change.

3. Final Answer:

   let x = 5; -> Option A

4. Quick Check:

   Use let/var/const without types [OK]

Hint: Use let or var without type annotations [OK]

Common Mistakes:

• Using type keywords like int or number in plain JavaScript
• Confusing const with let for variable declaration
• Trying to declare types explicitly in JavaScript

3. What will be the output of this code?

let value = 10;
value = 'hello';
console.log(typeof value);

medium

A. "number"

B. "undefined"

C. "string"

D. TypeError

Solution

1. Step 1: Track variable type changes

   Initially, value is a number 10, then reassigned to string 'hello'.

2. Step 2: Evaluate typeof operator

   typeof value returns the current type, which is "string" after reassignment.

3. Final Answer:

   "string" -> Option C

4. Quick Check:

   typeof after reassignment = "string" [OK]

Hint: typeof shows current type after reassignment [OK]

Common Mistakes:

• Assuming type stays as number after reassignment
• Confusing typeof output with variable value
• Expecting an error when changing types

4. Find the error in this code snippet:

let data = 5;
data = true;
console.log(data.toFixed(2));

medium

A. TypeError because boolean has no toFixed method

B. No error, outputs "5.00"

C. SyntaxError due to reassignment

D. ReferenceError for data variable

Solution

1. Step 1: Understand variable type changes

   Variable data starts as number 5, then changes to boolean true.

2. Step 2: Check method call on boolean

   toFixed is a method for numbers, not booleans, so calling it on a boolean causes a TypeError.

3. Final Answer:

   TypeError because boolean has no toFixed method -> Option A

4. Quick Check:

   Calling number method on boolean causes TypeError [OK]

Hint: Check method compatibility with current variable type [OK]

Common Mistakes:

• Assuming toFixed works on booleans
• Thinking reassignment causes syntax error
• Ignoring runtime errors from wrong method calls

5. You want to write a function that accepts any type of input and returns its type as a string. Which code correctly uses dynamic typing to achieve this?

function getType(input) {
  // Your code here
}

hard

A. return typeof input.toString();

B. return input.type;

C. return input.constructor;

D. return typeof input;

Solution

1. Step 1: Understand typeof operator

   typeof returns the type of any JavaScript value as a string.

2. Step 2: Evaluate each option

   return typeof input; correctly returns the type string. return input.type; tries to access a property that may not exist. return input.constructor; returns a function, not a string. return typeof input.toString(); returns "string" always because toString() returns a string.

3. Final Answer:

   return typeof input; -> Option D

4. Quick Check:

   typeof returns type string for any input [OK]

Hint: Use typeof to get type as string for any value [OK]

Common Mistakes:

• Using .type property which may be undefined
• Returning constructor function instead of string
• Calling toString() then typeof, always returns "string"