Node.js by Example: Functions

Functions are one of the core building blocks of JavaScript.

Here’s a function that takes two parameters and returns their sum.

 
function plus(a, b) {

JavaScript requires explicit returns. By default, a function will return undefined.

 
    return a + b;
}

Call a function with name(args).

 
let res = plus(1, 2)
console.log("1+2 =", res);

Because JavaScript does not define types, we can use the same function for multiple types.

 
res = plus("foo", "bar");
console.log("foo+bar=", res);

Function definitions may specify a default value. This is also known as an optional parameter

 
function plusPlus(a, b, c = 5) {
    return a + b + c
}

Optional parameters do not need to be specified but will be overridden when present

 
res = plusPlus(1, 2, 3);
console.log("1+2+3 =", res);
 
res = plusPlus(1, 2);
console.log("1+2+5=", res);

Function definitions are hoisted within the file. We can call a function before its definition.

 
hello();
 
function hello() {
    console.log("Hello");
}

Functions can be assigned as values, they can also be defined without a name. This makes them first class citizens.

 
const greet = function(name) {
    console.log("Howdy,", name);
}
greet("Chosen Undead");

Arrow Functions can be created using the => shorthand.

 
const print = (parameter) => {
    console.log(parameter);
}

The arguments keyword exists inside each function and contains an array-like list of the arguments passed in.

 
function printAll() {
    console.log(arguments);
}
printAll("a", "b", "c");

Rest parameters are the remaining arguments combined into an array

 
function restParams(one, two, ...rest) {
    console.log(one, two);
    console.log("rest:", rest);
}
restParams("a", "b", "c", "d");
 
$ node functions.js
1+2 = 3
foo+bar= foobar
1+2+3 = 6
1+2+5= 8
Hello
Howdy, Chosen Undead
[Arguments] { '0': 'a', '1': 'b', '2': 'c' }
a b
rest: [ 'c', 'd' ]

Next example: .

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