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# How This Book Works

A program is a list of instructions for a computer, written in a language precise enough that
the computer can follow it. This book teaches one such language, **TJS**, and it does that
by letting you run programs right here, on the page.


## Running an example

Here is a complete program. It puts a greeting on the page:

```tjs:inline {"view": "code"}
preview.append('Hello, world!')
```

The program is on the right and what it did is on the left. On a narrow device, such as a
phone, what it did appears above the program instead. `preview` is the box the program draws
in, and `append` adds something to it.

Now change it. Replace `world` with your own name. The page runs your new version as you
type, and the greeting changes. You cannot break anything: if you make a mistake, the
example tells you what went wrong, and you can fix it or undo it.

## Asking questions with the console

Some examples open with a **console** instead of a picture. A console answers questions: type
something at its prompt, press Enter, and it shows you the value.

```tjs:inline {"view": "console"}
console.log('The console shows what a program tells it.')
```

Try typing `2 + 3` at the prompt, then `'Hello, ' + 'world'`. You have just run two tiny
programs of your own.

## Checking your answers

Most examples end with a **test**: a few lines that check the program did what it should.

```tjs:inline {"view": "code"}
const greeting = 'Hello, world!'
preview.append(greeting)

test 'the greeting says hello' {
  expect(greeting).toBe('Hello, world!')
}
```

The line under the result says how the tests did: when they pass, how many passed. Change
the greeting and the test fails: the line turns red and says what it expected and what it
got. Click it for the details. That is the whole loop of this book: run something, change
it, and let the tests tell you whether you understand what happened.

If you are curious how these live examples work, the
[tosijs-ui live-example documentation](https://ui.tosijs.net) explains the machinery. You do
not need it to learn to program.