Control Flow: Making Decisions with Conditionals
Learn how to make decisions in your code using if, else, and switch
Programs rarely run the same code every time. Conditionals let your code make decisions based on data.
The if Statement
An if block runs only when its condition is true:
let temperature = 30;
if (temperature > 25) {
console.log("It's a hot day!");
}The condition inside parentheses is evaluated as a boolean. If it is true, the block runs.
if...else
Use else to run code when the condition is false:
let age = 17;
if (age >= 18) {
console.log("You are an adult.");
} else {
console.log("You are a minor.");
}if...else if...else
Chain multiple conditions with else if:
let score = 85;
if (score >= 90) {
console.log("Grade: A");
} else if (score >= 80) {
console.log("Grade: B");
} else if (score >= 70) {
console.log("Grade: C");
} else {
console.log("Grade: F");
}Conditions are checked top to bottom. Once one matches, the rest are skipped.
Comparison Operators
Operator | Meaning | Example |
|---|---|---|
== / === | Equal (loose / strict) | 5 === 5 |
!= / !== | Not equal | 5 !== "5" |
> | Greater than | 10 > 5 |
< | Less than | 5 < 10 |
>= | Greater than or equal | 10 >= 10 |
<= | Less than or equal | 5 <= 10 |
Always prefer === over ==: strict equality checks both value and type.
Logical Operators
Combine conditions with logical operators:
let age = 25;
let hasLicense = true;
if (age >= 18 && hasLicense) {
console.log("You can drive.");
}
if (age < 18 || !hasLicense) {
console.log("You cannot drive.");
}&&: both must be true||: at least one must be true!: inverts a boolean
The switch Statement
Use switch when comparing a single value against many options:
let day = 3;
switch (day) {
case 1:
console.log("Monday");
break;
case 2:
console.log("Tuesday");
break;
case 3:
console.log("Wednesday");
break;
default:
console.log("Unknown day");
}The break prevents fall-through to the next case.
Ternary Operator
A concise shorthand for simple if...else:
let age = 20;
let status = age >= 18 ? "Adult" : "Minor";
console.log(status); // Output: AdultSyntax: condition ? valueIfTrue : valueIfFalse
Practice Exercise
Write a function that checks if a number is positive, negative, or zero:
function checkNumber(n) {
if (n > 0) {
return "Positive";
} else if (n < 0) {
return "Negative";
} else {
return "Zero";
}
}
console.log(checkNumber(10)); // Output: Positive
console.log(checkNumber(-5)); // Output: Negative
console.log(checkNumber(0)); // Output: ZeroSummary
Construct | Use |
|---|---|
if (cond) { ... } | Run code when condition is true |
if ... else | Choose between two paths |
if ... else if ... else | Choose between multiple paths |
switch (val) { case: ... } | Match value against many options |
cond ? a : b | Inline conditional expression |
Next Steps
Now that your code can make decisions, the next lesson shows how to repeat actions with loops.