Values and Names

Programs work with values: numbers, text, true and false. This chapter is about the kinds of values there are, and how to give a value a name so you can use it again.

Kinds of value

Ask the console. Type each of these at its prompt and press Enter:

console.log('Type 7 * 6, then 10 / 4, then 7 > 3 at the prompt below.')

• 7 * 6 is 42. Numbers do arithmetic: +, -, * (multiply) and / (divide).
• 10 / 4 is 2.5. Numbers can have a fractional part.
• 7 > 3 is true. A comparison is a question, and its answer is a boolean: true or false.

So far you have met three kinds of value: numbers, strings and booleans. typeof tells you which kind a value is; try typeof 42, typeof 'hi' and typeof true at the prompt.

Naming a value

const gives a value a name:

const width = 6
const height = 7
const area = width * height

preview.append(`A ${width} by ${height} rectangle has an area of ${area}.`)

test 'area is width times height' {
  expect(area).toBe(42)
}

Once a value has a name, you can use the name wherever you would use the value. Change width to 10 and look at the sentence, then at the test. Why does the test fail now, and what should it expect instead?

Names that change

A const name keeps its value. When a value needs to change, use let:

let score = 0
score = score + 10
score = score + 5

preview.append(`Your score is ${score}.`)

test 'the score adds up' {
  expect(score).toBe(15)
}

score = score + 10 reads "set score to what score was, plus 10". Use const unless you need the name to change; then a reader knows at a glance that the value stays put.

Asking whether two values are the same

== asks whether two values are equal, and != asks whether they differ:

const answer = 42
const guess = 40 + 2

preview.append(`Is the guess right? ${guess == answer}`)

test 'comparing values' {
  expect(guess == answer).toBe(true)
  expect('42' == 42).toBe(false)
}

The last line checks that the text '42' is not the number 42. They look alike, but they are different kinds of value, and TJS keeps them apart. (Some other languages quietly treat them as equal, which causes a whole family of bugs. TJS does not.)

Try it

Write a program that names your age, names the current year, works out the year you were born, and shows a sentence about it. Add a test that checks the birth year. Then change your age and watch the test tell you to update it.