Conditions with if - JavaScript

if runs a block of code when its condition is true. Add else if and else to handle the other cases.

JavaScript

const hour = 21;

if (hour < 12) {
  console.log("Good morning");
} else if (hour < 18) {
  console.log("Good afternoon");
} else {
  console.log("Good evening");
}

Output

Good evening

How the greeting example above picks a branch

Use === for equality

JavaScript has two equality operators. == converts types before comparing, which gives surprising results. === (strict equality) compares both the value and the type, so prefer it.

JavaScript

Open in playground

console.log(5 == "5");
console.log(5 === "5");
console.log(0 == false);
console.log(0 === false);

Output

true
false
true
false

Truthy and falsy values

A condition does not have to be a boolean. The values false, 0, "" (empty string), null, undefined and NaN count as falsy; everything else is truthy.

JavaScript

Open in playground

const username = "";

if (username) {
  console.log(`Welcome back, ${username}!`);
} else {
  console.log("Please log in.");
}

Output

Please log in.

Combining conditions

Use && (and), || (or) and ! (not). The ternary operator condition ? a : b picks one of two values.

JavaScript

Open in playground

const age = 16;
const withParent = true;

const canWatch = age >= 18 || withParent;
console.log(canWatch ? "Enjoy the movie" : "Sorry, adults only");
console.log(!canWatch);

Output

Enjoy the movie
false