Conditions

Programs make decisions. Conditions let your code take one path or another.

if

if (condition) {
  // runs only when the condition is true
}

if and else

<script>
const hour = new Date().getHours();

if (hour >= 9 && hour < 17) {
  console.log("The shop is open.");
} else {
  console.log("Sorry, we're closed. Open 9am to 5pm.");
}
console.log("It is now hour", hour);
</script>

else if: more than two choices

Chain conditions with else if. JavaScript checks them in order and runs the first one that’s true.

<script>
const marks = 74;
let grade;

if (marks >= 80) {
  grade = "A+";
} else if (marks >= 70) {
  grade = "A";
} else if (marks >= 60) {
  grade = "A-";
} else if (marks >= 50) {
  grade = "B";
} else if (marks >= 40) {
  grade = "C";
} else if (marks >= 33) {
  grade = "D";
} else {
  grade = "F";
}

console.log(marks + " marks is grade " + grade);
</script>

Change marks and run it again. Then try moving the marks >= 40 check to the top. Every passing mark becomes a C, because the first true condition wins. Order matters.

True-ish and false-ish values

A condition doesn’t have to be a comparison. JavaScript treats some values as false:

false, 0, "" (an empty string), null, undefined and NaN.

Everything else counts as true. That gives a handy check for an empty input:

const name = "";
if (name) {
  console.log("Hello " + name);
} else {
  console.log("Please type your name");
}

The short form

For a simple either-or value, the ternary operator fits on one line:

const label = age >= 18 ? "Adult" : "Child";

Read it as: is age at least 18? If so, “Adult”, otherwise “Child”. Keep it for short choices. For anything bigger, if is easier to read.