Javascript · Variables
The typeof Operator
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — What is the typeof Operator?
When you're coding, sometimes you need to check what type of data you're working with. Is it a number? A string? A boolean? JavaScript gives us a special tool for this: the
typeof
operator!
The
typeof
operator tells you the type of any value. Here's how to use it:
// Basic syntax
console.log(typeof 42);
console.log(typeof "Hello");
console.log(typeof true);
// You can also use parentheses
console.log(typeof(42));
Notice that
typeof
always returns a string that describes the type. For example,
typeof 42
returns the string
"number"
, not the number itself!
Why do we need typeof?
JavaScript is a dynamically typed language, meaning variables can hold different types of values. The typeof operator helps you check what type you're working with before performing operations. This is especially useful when you receive data from users or other sources and need to validate it!
Exercise 2 — typeof with Different Types
Let's explore what
typeof
returns for each primitive type you've learned! Try running this code and experiment by changing the values:
let codemonName = "Aquachu";
let codemonLevel = 25;
let isWaterType = true;
let specialMove;
console.log("Name type:", typeof codemonName);
console.log("Level type:", typeof codemonLevel);
console.log("Water type check:", typeof isWaterType);
console.log("Special move type:", typeof specialMove);
Here's what
typeof
returns for each type:
-
"string"
for text values
-
"number"
for numeric values (integers and decimals)
-
"boolean"
for true/false values
-
"undefined"
for variables that haven't been assigned a value
Exercise 3 — Predicting typeof Results
What will the following code output?
let codemonHP = 100;
console.log(typeof codemonHP);
Options
• ```"number"```
• ```boolean```
• ```"integer"```
• ```100```
Hint : Remember: ```typeof``` always returns a string that describes the type. The value 100 is a numeric value.
Exercise 4 — Using typeof Operator
Complete the code to check the type of the
codemonName
variable and log it to the console:
let codemonName = "Pyrochu";
console.log(____ ____);
Options: typeof typeOf codemonName "codemonName" type checkType
Hint : The ```typeof``` operator is followed by the variable name (without quotes). Use ```console.log()``` to display the result.
Exercise 5 — Match Values to Their typeof Results
Match each value with what
typeof
would return for it:
Items to match
• "Florachu"
• 42.5
• false
• undefined
• 0
Definitions
• "string"
• "number"
• "boolean"
• "undefined"
• "number"
Exercise 6 — Understanding typeof Return Value
What is the type of the value that
typeof
returns?
let result = typeof 42;
console.log(typeof result);
Options
• ```"string"```
• ```"number"```
• ```"type"```
• ```"undefined"```
Hint : The ```typeof``` operator always returns a string that describes the type. Even when you check the type of the result, it's a string!
Exercise 7 — Display Variable Types
Practice using
typeof
to check variable types!
Your task:
1. Create a variable
codemonName
with a string value
2. Create a variable
codemonLevel
with a number value
3. Create a variable
isCodemonAlive
with a boolean value
4. Use
typeof
and
console.log()
to display the type of each variable
Starter code
// Create your variables and display their types here