5. Functions

Defining and calling

func add(a, b) {
    return a + b;
}
print(add(2, 3));   // 5

A function with no return returns null.

Parameters

Parameters are positional. Type annotations are allowed (and ignored):

func greet(name: string, times: int) {
    let i = 0;
    while (i < times) {
        print("Hello " + name);
        i = i + 1;
    }
}
greet("Ada", 2);

Recursion

Functions can call themselves:

func factorial(n) {
    if (n <= 1) { return 1; }
    return n * factorial(n - 1);
}
print(factorial(5));   // 120

func fib(n) {
    if (n <= 1) { return n; }
    return fib(n - 1) + fib(n - 2);
}
print(fib(10));   // 55

Arrow functions (single expression)

For short, one-expression functions use =>:

square(x) => x * x;
double(x) => x * 2;
print(square(6), double(6));   // 36 12

Functions are values

You can pass a function to another function (used by the web server in lesson 12):

func apply(f, x) {
    return f(x);
}
square(n) => n * n;
print(apply(square, 9));   // 81

Scope

Variables created inside a function are local to it. Use return to send a result out.

func makeCounterStart() {
    let count = 100;
    return count;
}
print(makeCounterStart());   // 100

Next: Collections