TypeScript Booleans

true or false

A boolean stores one of two values: true or false. That is the whole type. You use it to remember a yes-or-no fact: a door is open, a score is high enough, a loop should keep going.

Write the literals in lowercase. True and False are Python. TypeScript will not compile them unless you declared names with those spellings — and you should not.

A complete program

Declare a boolean, print it, then print a comparison. console.log prints the wordstrue and false, not 1 and 0 the way C++ coutoften does.

const ready: boolean = true;
const empty: boolean = false;
console.log(ready);
console.log(empty);
console.log(7 > 3);
console.log(7 === 3);

Comparisons produce booleans

Every comparison expression has type boolean. You can store it, print it, or pass it straight intoif. Prefer === for equality, as in the Operators chapter.

Combine with and, or, not

TypeScript uses && (and), || (or), and ! (not).&& is true only if both sides are true. || is true if at least one side is true. ! flips a value.

const age: number = 20;
const member: boolean = true;
console.log(age >= 18 && member);
console.log(age < 18 || member);
console.log(!member);

Store a comparison

You do not have to print a comparison immediately. Put it in a boolean variable and reuse the name. Change age and compile again to see the stored value flip.

const age: number = 20;
const adult: boolean = age >= 18;
console.log(adult);
console.log(adult === true);

Conditions consume them

An if, a while, and the middle part of a for all expect something that converts to boolean. You will write that in the next chapters. The value itself is still just true or false.

const ready: boolean = true;
if (ready) {
  console.log("go");
} else {
  console.log("wait");
}

Other values convert too: 0, "", null, and undefined are treated as false in a condition. Prefer an actual boolean or a comparison so the intent is obvious.

What to remember

• boolean holds true or false.
• Comparisons such as === and < produce a boolean.
• console.log prints the words true and false.
• Combine tests with &&, ||, and !.

Next: if and else — the first place those booleans get used.