Type checking using typeof in Javascript - Step-by-Step Execution

Concept Flow - Type checking using typeof

Start

↓

Evaluate expression

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Apply typeof operator

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Return type as string

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Use type info (e.g., if, print)

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End

The program evaluates a value, applies the typeof operator to get its type as a string, then uses that type information.

Execution Sample

Javascript

let x = 42;
console.log(typeof x);

let y = 'hello';
console.log(typeof y);

This code checks the types of variables x and y and prints them.

Execution Table

Step	Expression	typeof Result	Action	Output
1	x = 42	number	Assign 42 to x
2	typeof x	number	Check type of x	number
3	console.log(typeof x)	number	Print type of x	number
4	y = 'hello'	string	Assign 'hello' to y
5	typeof y	string	Check type of y	string
6	console.log(typeof y)	string	Print type of y	string
7			End of code

💡 All expressions evaluated, program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 4	Final
x	undefined	42	42	42
y	undefined	undefined	'hello'	'hello'

Key Moments - 2 Insights

Why does typeof return a string like "number" instead of the value itself?

Because typeof always returns the type of the value as a string, not the value. See execution_table step 2 and 5 where typeof x and typeof y return "number" and "string".

What happens if we use typeof on a variable that is not defined?

Using typeof on an undeclared variable returns "undefined" without error. This is safe unlike directly accessing the variable. This is not shown here but is important to know.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of typeof x at step 2?

A "string"

B "number"

C 42

D undefined

💡 Hint

Check the 'typeof Result' column at step 2 in the execution_table.

At which step is the variable y assigned a value?

A Step 4

B Step 5

C Step 1

D Step 6

💡 Hint

Look at the 'Action' column for assignment actions in the execution_table.

If we changed x to true, what would typeof x return?

A "string"

B "number"

C "boolean"

D "object"

💡 Hint

Recall typeof returns the type as a string; true is a boolean type.

Concept Snapshot

typeof operator returns the type of a value as a string.
Syntax: typeof value
Common results: "number", "string", "boolean", "undefined", "object", "function".
Useful for checking variable types before using them.
Safe to use on undeclared variables (returns "undefined").

Full Transcript

This visual execution shows how the typeof operator works in JavaScript. First, variables x and y are assigned values 42 and 'hello'. Then, typeof is applied to each variable, returning their types as strings: "number" for x and "string" for y. These results are printed to the console. The variable tracker shows how x and y change from undefined to their assigned values. Key moments clarify that typeof returns a string describing the type, not the value itself, and that using typeof on undeclared variables is safe. The quiz tests understanding of the typeof results and variable assignments.