Project: rock, paper, scissors in JavaScript

What you will build

The classic rock, paper, scissors against the computer, best of 5. Great for mastering Math.random(), if / else if and keeping state (the score) between plays.

Play here — it is the page's real JavaScript:

Step 1: the computer's move

Three options in an array. Math.random() gives a number between 0 and 1; we multiply it by 3 and round down to get an index (0, 1 or 2).

const OPCIONES = ["piedra", "papel", "tijera"];

function jugadaOrdenador() {
    const indice = Math.floor(Math.random() * 3);   // 0, 1 o 2
    return OPCIONES[indice];
}

Step 2: who wins

There are only three ways to win. If it's not a draw and you don't win, the machine wins.

function gana(a, b) {
    return (a === "piedra" && b === "tijera") ||
           (a === "papel"  && b === "piedra") ||
           (a === "tijera" && b === "papel");
}

Step 3: one round

We compare, update the score and show it.

let puntosTu = 0;
let puntosCpu = 0;

function ronda(tuJugada) {
    const cpu = jugadaOrdenador();

    if (tuJugada === cpu) {
        console.log("Empate");
    } else if (gana(tuJugada, cpu)) {
        puntosTu++;
        console.log("Ganas la ronda");
    } else {
        puntosCpu++;
        console.log("Gana la CPU");
    }

    console.log(`Marcador: ${puntosTu} - ${puntosCpu}`);
}

Step 4: game over and buttons

Best of 5: the first to reach 5 wins. We wire each button to ronda().

const META = 5;

function comprobarFinal() {
    if (puntosTu === META)  console.log("🏆 ¡Ganas la partida!");
    if (puntosCpu === META) console.log("💥 Gana la CPU");
}

document.querySelectorAll("#ppBotones button").forEach(boton => {
    boton.addEventListener("click", () => {
        if (puntosTu < META && puntosCpu < META) {
            ronda(boton.dataset.op);
            comprobarFinal();
        }
    });
});

Where to go next

• Add "lizard" and "Spock" (rock-paper-scissors-lizard-Spock).
• Save the best streak in localStorage.
• Animate the computer's hand before showing the result with setTimeout.
• Make the computer "learn": have it play whatever beats your most repeated move.