Function

In Node.js, we declare a function with the function keyword or the arrow syntax. Functions can call other functions—this is called structural programming.

We can pass functions as arguments to other functions. These are higher-order functions. Often, simpler code with for-loops is faster.

First example

Node programs are full of functions. When we see console.log, this too is a function call—one we do not need to define ourselves.

function multiplyBy2(value) {
    // Multiply the argument by 2.
    return value * 2;
}

// Call the function.
console.log("FUNCTION RESULT: " + multiplyBy2(4));
FUNCTION RESULT: 8

Function argument

We can create an anonymous function, and pass it as an argument to another function. Here we invoke forEach, which loops over the elements of an array.

// Print 3 integer elements with an anonymous function.
// ... Pass the function as an argument to forEach.
[10, 20, 30].forEach(function(x) {
    console.log("VALUE IS: " + x);
});
VALUE IS: 10
VALUE IS: 20
VALUE IS: 30

Return

A function can return a value. Often we use if-statements to branch inside a function, and then return the correct value. A return is not required for all functions.

function getWord(n) {
    // Return string representation of number.
    if (n === 1) {
        return "one";
    }
    else if (n === 2) {
        return "two";
    }
    else {
        return "unknown";
    }
}

console.log("GETWORD: " + getWord(1));
console.log("GETWORD: " + getWord(2));
console.log("GETWORD: " + getWord(1000));
GETWORD: one
GETWORD: two
GETWORD: unknown

Return, void function

Suppose we try to use the return value of a function, but no return value is reached. We get the special value "undefined."

function getWord(n) {
    if (n === 1) {
        return "one";
    }
}

// If no return statement is reached, we get undefined.
console.log("RESULT: " + getWord(1000));
RESULT: undefined

Arrow function

Early versions of JavaScript do not have arrow functions. But browser support for arrow functions has emerged. We have a left and a right side separated by the arrow operator.

// Use an arrow function and store the function in a variable.
var pets = (cats, dogs) => (cats + dogs)
// Call arrow function.
var result = "I have " + pets(1, 2) + " pets.";

console.log("ARROW FUNCTION: " + result);
ARROW FUNCTION: I have 3 pets.

Optimization, function reduction

JavaScript functions are objects—this means they are often allocated and garbage-collected. Invoking the garbage collector tends to be slow in languages.

function single1() {
    console.log("RESULT 1");
}
function single2() {
    console.log("RESULT 2");
}

function combo(x) {
    if (x === 0) {
        console.log("RESULT 3");
    }
    else {
        console.log("RESULT 4");
    }
}

// Call functions.
single1();
single2();
combo(0);
combo(1);
RESULT 1
RESULT 2
RESULT 3
RESULT 4

Multiple return values

A JavaScript program can return multiple values from a function. An array or object (passed by a reference, or created in the function) can be used.

Lookup table

We can develop a lookup table of functions. Then we can skip if-statements, and access functions based on a value. This requires some planning.

With functions we add structural designs to our Node programs. And with arrow functions, we have a simple syntax for lambda expressions.