The boolean Type

Boolean values are true or false and commonly represent flags, comparisons and the outcomes of logical expressions.

In this lesson, you will study the rule, run a complete TypeScript example, explain its output, and test a small change.

Annotate booleans with boolean, not Boolean. Comparisons such as stock > 0 produce booleans. An if statement can also interpret non-boolean truthy and falsy values, but explicit boolean conditions often make intent clearer.

Save this code as main.ts or paste it into the TypeScript Playground. The example uses fixed values so it runs without prompting for input.

TypeScript code

const stock: number = 3;
 const available: boolean = stock > 0;
 console.log(available);
 console.log(!available);

Verified output

true
false

How to run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. When using a project configuration, run the matching project build instead.

Because 3 is greater than 0, available becomes true. Logical NOT (!) reverses it to false.

Predict the output before you run the sample. Compare each line with the source expression that produced it, and distinguish the type checker’s work from what happens in JavaScript at runtime.

Boolean("false") returns true because nonempty strings are truthy. Use a real comparison or parser for text input.

Debugging habit: when a change is rejected, read the exact compiler diagnostic and inspect the operand types. When it compiles but behaves unexpectedly, follow the values through the JavaScript expressions.

Set stock to 0. Predict available and !available, then check the output.

Challenge: write down your prediction first, run the changed source, and explain any difference. Return to the verified version before continuing.

The boolean Type: Annotate booleans with boolean, not Boolean.

A comparison produces true or false.

Before continuing, be able to explain what is checked before execution and what actually changes at runtime in the example above.