Truthy and Falsy

Here's something interesting. The condition inside an if doesn't have to be an actual boolean. You can put any value in there, and JavaScript will decide whether it counts as "true-ish" or "false-ish."

Values that count as false are called falsy. There are only eight of them, so you can actually memorize this list:

Everything else is truthy. Every non-empty string, every number except zero, every list, every object. All truthy.

if ("hello") {
  console.log("Non-empty strings are truthy");
}

if (42) {
  console.log("Non-zero numbers are truthy");
}

if ("") {
  console.log("You will never see this");
}

if (0) {
  console.log("Or this");
}

Non-empty strings are truthy
Non-zero numbers are truthy

You can see whether something is truthy or falsy by converting it with Boolean():

console.log(Boolean("pizza"));
console.log(Boolean(""));
console.log(Boolean(0));
console.log(Boolean(-5));
console.log(Boolean(null));

true
false
false
true
false

Why is this useful?

It lets you write short, readable checks. For example, "did the user actually type something?"

const username = "";

if (username) {
  console.log(`Hello, ${username}!`);
} else {
  console.log("Please enter a username.");
}

Please enter a username.

Since the empty string is falsy, if (username) means "if the username isn't empty." Neat!