Function basics and calls

A function is reusable code that performs one clear task. Once steps are placed in a function, calling its name runs them again without copying the whole block.

Defining and calling a function

After function comes the function name. Parameters go inside parentheses and the function body goes inside braces. The actual values supplied by a call are arguments.

function larger(num1, num2) {
    if (num1 > num2) {
        return num1;
    }
    return num2;
}

const result = larger(7, 12);
console.log(result); // 12

return sends a result back to the calling position and immediately ends that call.

Functions without an explicit result

Some functions only perform an action. They can omit return; reaching the end of the body returns undefined.

function printGreeting(name) {
    console.log(`Hello, ${name}!`);
}

printGreeting("Mina");

Calling another function

One function can call another. Execution enters the called function, receives its return value, and then continues at the original calling position.

function square(value) {
    return value * value;
}

function distance(x1, y1, x2, y2) {
    const dx = x2 - x1;
    const dy = y2 - y1;
    return Math.sqrt(square(dx) + square(dy));
}

console.log(distance(0, 0, 3, 4)); // 5

A JavaScript file executes from the top. You can declare functions and call them where needed; the language does not automatically look for a function named main.