Umbrella or Not

Asking a question before acting

Everything you have written so far ran once, top to bottom. Right for a banner, useless for advice, because advice depends on the day.

if is how a program asks before it acts. The question goes in round brackets; the lines to run when the answer is yes go in curly braces straight after. Add else and a second pair of braces for the no. Exactly one block runs — never both, never neither.

The braces are what matter: they alone decide which lines belong to the question.

if (somethingIsTrue) {
  // inside the block
}
// outside it, and runs either way

JavaScript ignores the spaces at the start of a line. You indent because it makes the shape obvious to a human; the braces are the only thing JavaScript reads. If you have met Python, that is the exact reverse. Indent for yourself, brace for JavaScript.

One equals sign, two, or three

Inside the round brackets goes a comparison. === asks whether two values are the same, and >= asks whether the left one is at least as big as the right.

One equals sign is not on that list. That is the trap.

• = stores. const rainChance = 70; puts 70 under that name. It changes something.
• === compares. rainChance === 70 asks whether what is under that name happens to be 70. It changes nothing.

Write one where you meant three and JavaScript tries to store. Both values here were made with const, so it stops with TypeError: Assignment to constant variable. — and only after the card above has printed, because JavaScript runs down the page until it hits trouble.

A third form, ==, compares loosely: it converts one side first, so the text "50" counts as equal to the number 50. === checks the value and the kind of value together, and says no. This site uses === everywhere, because fewer surprises beats a shorter line.

A worked example

A lift rather than a weather report, so the answer here stays yours to write:

const floor = 3;
const groundFloor = 0;

console.log("Lift is on floor", floor);

if (floor === groundFloor) {
  console.log("Doors open to the street.");
} else {
  console.log("Going down first.");
}

console.log("Lift check done.");

Lift is on floor 3
Going down first.
Lift check done.

3 is not 0, so the first block is skipped whole and the second one runs. The last line sits outside both pairs of braces, so it prints either way. Change floor to 0 and the middle line switches to the other block's, while Lift check done. prints exactly as before.

Your turn

The editor holds a rain report. rainChance is today's forecast; umbrellaMark is the point at which an umbrella is worth carrying. Two-word names in JavaScript are written like that: second word capitalised, no gap.

Press Run first. The card appears, then advises leaving the umbrella at home on a day with a seventy percent chance of rain, because the question currently reads:

if (rainChance === 0) {

which asks whether the chance is exactly zero: a fair question, and nothing to do with umbrellas.

Replace it with one asking whether rainChance has reached umbrellaMark: at the mark or above counts as take it. Leave the braces and everything inside them as they are.

Then change rainChance to 20 and run it again. The same program should now send you out without one — proof the advice is worked out from the numbers rather than written in.

If something goes wrong

The likeliest slip is the equals signs. With one, the card prints and the program then stops on TypeError: Assignment to constant variable. — JavaScript was told to store into a name const had fixed. Count them; a question needs three.

Next is a brace. Drop the { from the if line and only the next single line counts as the block, which leaves the } further down closing nothing: JavaScript stops with SyntaxError: Unexpected token '}' before running a thing. Take that } away too and the complaint moves to SyntaxError: Unexpected token 'else'. Braces come in pairs.

If the card prints but the advice is wrong, nothing is broken. Read your question out loud as a sentence and check it is the one you meant to ask.