> For the complete documentation index, see [llms.txt](https://cs.brash.ca/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cs.brash.ca/unit-4/user-defined-functions.md).

# User Defined Functions

Almost all programming languages allow us to write our own functions. In Javascript, the syntax is:\
`function name (params) {`\
    `// Your code with optional return value`\
`}`

Let's say we need to do something over and over in our program - perhaps decide how many zeros are in a quadratic equation given `a` ,`b` ,and `c`. Rather than do the math over and over, we can create a function and return either 0, 1, or 2.

```javascript
// Determine how many real roots exist
function descriminate(a, b, c) {
    let desc = Math.pow(b, 2) - (4*a*c);

    if (desc < 0)
        return 0;
    else if (desc === 0)
        return 1;
    else
        return 2;
}
```

This would allow us to make decisions or act on the return value of 0, 1, or 2. We could print it, use it for other math, or ignore it completely.

### Ex. 1 - No return value

If your function does not have a `return` call with a value, the result is `undefined`. Here's a simple example of a function that doesn't return anything:

{% embed url="<https://tech.io/snippet/X5Qzbt5>" %}

### Ex. 2 - No parameters

It isn't often that a user-defined function has zero parameters. However, it can be done. Here's one that *shouts* the current time (it prints it to the console). No parameters, no return value.

{% embed url="<https://tech.io/snippet/uBaFDXd>" %}

### Ex. 3 - Parameters

Here's a function that takes two parameters, `base` and `exponent`. It prints the answer to the console. Again, no return value.

{% embed url="<https://tech.io/snippet/Dw76prs>" %}

### Ex. 4 - *Optional* Parameters

In example 2, [above](/unit-4/user-defined-functions.md#ex-2-no-parameters), the time is output in 24-hour "military" time. Perhaps we only want it to do that if we ask it to. We can have a parameter `militaryTime` that is false by default but if we pass `true` as a parameter to the function, it prints in 24-hour format.

{% embed url="<https://tech.io/snippet/nAPp6P8>" %}

Notice how we defined the parameter to our function: `shoutCurrentTime(militaryTime = false)` \
This defines the parameter and sets it to `false`, unless we set it to something else.

### Ex. 5 - Return Values

The true strength of a function is the ability to return an answer. It could be a modification of the parameter(s) passed into the function or a completely unrelated piece of data. The `return` keyword breaks out of the function, no matter what is happening, and sends whatever data you want to return.\
**Note:** if you need to return more than one piece of data, you must return it in an [array](/tl-dr/basics.md#arrays) or object.

Many examples in one:

{% embed url="<https://tech.io/snippet/RQou9lC>" %}