Dynamic typing in JavaScript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does dynamic typing mean in JavaScript?

Dynamic typing means variables can hold any type of value and their type can change during the program's execution.

beginner

How do you declare a variable in JavaScript that can hold any type of value?

You can use let or var to declare a variable without specifying its type, allowing it to hold any value.

beginner

What happens when you assign a number to a variable and then assign a string to the same variable?

The variable changes its type from number to string because JavaScript variables are dynamically typed.

beginner

Show an example of dynamic typing in JavaScript.

Example:<br><pre>let value = 10;  // value is a number
value = "hello";  // now value is a string</pre>

beginner

Why is dynamic typing useful in JavaScript?

It makes coding faster and more flexible because you don't have to declare variable types explicitly.

What type can a JavaScript variable hold?

A Only strings

B Only numbers

C Any type, and it can change during execution

D Only one type fixed at declaration

JavaScript variables are dynamically typed, so they can hold any type and change types.

Which keyword allows you to declare a variable that can change type in JavaScript?

A let

B int

C string

D const

let declares a variable that can hold any type and can be reassigned.

What happens if you assign a string to a variable that previously held a number?

A The variable keeps the number

B The variable changes to hold a string

C An error occurs

D The program crashes

In dynamic typing, the variable's type changes to the new assigned value's type.

Which of these is NOT a benefit of dynamic typing?

A No need to declare types explicitly

B More flexible variable use

C Faster coding

D Automatic type safety

Dynamic typing does not guarantee type safety; errors can happen at runtime.

Which of these is an example of dynamic typing?

A let x = 5; x = 'five';

B int x = 5; x = 'five';

C const x = 5; x = 10;

D string x = 'hello'; x = 10;

In JavaScript, let x = 5; x = 'five'; shows a variable changing type dynamically.

Explain dynamic typing in JavaScript with an example.

Think about how a variable can start as a number and then hold a string.

What are the advantages and possible downsides of dynamic typing in JavaScript?

Consider both why it helps and what problems it might cause.

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"