functions.md

Functions

A function is a reusable piece of code. Functions are very important in JavaScript, to the extent that some people call JavaScript a "function-oriented" language. As mentioned above, variables can be of type function. In fact, every function is a variable.

The following two pieces of code have the exact same result:

function sum(a, b) {
  return a + b;
}

and

let sum = function (a, b) {
  return a + b;
}

Note

This is not entirely true, as in the second code, the function is "anonymous", i.e. it has no name. But in both cases, you can call the function like this: sum(4, 5).

Parameters & arguments

When writing function sum(a, b), a and b are the "parameters" of the function. We say that this function has two parameters. (Sometimes, you'll see the word "arity": this function has "arity" 2, but that is something you don't have to use for now.)

Now, when calling function sum, e.g. var s = sum(4, 5);, we say that the numbers 4 and 5 are the "arguments" of the function. Arguments are "passed" to the function: "we pass 4 and 5 to the function sum".

So remember the difference between the word "parameter" and "argument". Many people confuse them, and that's not a big problem, but understanding the difference is always nice:

• A parameter is the name you want to give to the variable that is available inside of the function.
• An argument is the actual value you want to assign to the parameters when you call the function.

A function that "has two parameters" is also said to "take/accept two arguments". But, sometimes you'll hear people say: "the function has two arguments" or "the function takes two parameters". While formally incorrect, you'll know what they mean.

Calling a function on something

In JavaScript, you can call functions on something. By this, we mean that you use the dot to call the function. For instance, when we say "call method trim on string s", we mean:

let s = " this is a string  ";
s.trim(); // -> "this is a string"

Note

Technically, this means that the string s will become the this special variable inside of the function.

However, there are functions that you don't call on anything:

function sum(a, b) { return a + b; }
sum(4, 5); // -> 9

Here, you call the function sum on nothing.

Most built-in functions in JavaScript, like math functions or logging functions, also use the dot:

Math.round(4.5);
console.log("hello");
Array.from([1, 2, 3]);

Indeed, these functions are also called "on" Math, console, Array, and so on. However, in this case, their purpose is more to group them logically, so here it's not very important to use that terminology. We'd rather say: "call the function Math.round with 4.5 as an argument", i.e. we include it in the full name of the methods.

It's more when you think about which functions you can call "on" your own variables (strings, arrays, numbers, etc):

myString.trim();
myArray.slice();
myNumber.toString();
...