Making decisions with if

What you'll learn

How to compare values, and how to run code only when a condition is true.

Why it matters

Programs constantly decide things: is the password right, is the cart empty, is it past midnight. if is how code chooses a path.

Explanation

A condition is something that is either true or false. Comparisons produce conditions:

• === equal to, !== not equal to
• > greater than, < less than, >=, <=

An if statement runs its block only when the condition is true. Add else for the other case, and else if for more choices.

You can combine conditions: && means AND (both must be true), || means OR (at least one), ! means NOT.

Example

Step-by-step walkthrough

1. temperature is 28.
2. First check: is 28 greater than 30? No, skip that block.
3. Second check: is 28 greater than or equal to 20? Yes. It prints Nice weather.
4. The remaining else is skipped because an earlier branch ran.
5. The last line asks 17 >= 18 (false) AND true. AND needs both, so it prints false.

Try it yourself

Change temperature to 35, then to 10, and run each time. Which message appears?

if / elseJavaScript

Editable codeconst temperature = 28; if (temperature > 30) { console.log("It's hot. Drink water."); } else if (temperature >= 20) { console.log("Nice weather."); } else { console.log("Bring a jacket."); }

Run ResetCopyCtrl+Enter to run

Common mistakes

• Using = instead of ===. = assigns, === compares.
• Forgetting the curly braces around multi-line blocks.
• Wrong order of else if. Put the most specific check first or a broader one will catch it.
• Comparing strings and numbers. "5" === 5 is false because the types differ.

Quick quiz

Summary

• Comparisons make true/false conditions.
• if, else if, else choose which block runs.
• Combine conditions with &&, ||, !.

Next: repeating work with loops.

if (condition) {
  // runs when true
} else {
  // runs when false
}

const temperature = 28;

if (temperature > 30) {
  console.log("It's hot. Drink water.");
} else if (temperature >= 20) {
  console.log("Nice weather.");
} else {
  console.log("Bring a jacket.");
}

const age = 17;
const hasTicket = true;
console.log(age >= 18 && hasTicket);