## Defining Function

To customize a function's behavior, put variable names in the parentheses, which are called parameters. Example of a function that can add any two numbers:
```function add(num1, num2) {

}
```
To have the function return a result, use the keyword `return`. Example of returning the sum of two numbers:
```function add(num1, num2) {
return num1 + num2
}
```
When calling a function, pass in arguments for the parameters, and store the return value in a variable. Example of calling the function in the example above:
```let sum = add(2, 3)
```

### Exercise 1 of 6

Call the add function and store the return value in sum.

HTML

```<button id="sumButton">Show sum</button>
<p id="sumParagraph">Sum:</p>
```

JavaScript

```let sumButton = document.getElementById(`sumButton`)
let sumParagraph = document.getElementById(`sumParagraph`)

function showSum() {
// your code will appear here
sumParagraph.innerHTML = `Sum: \${sum}`
}

return num1 + num2
}
```

Sum:

### Exercise 2 of 6

Write the parameters for the subtract function so that the result is 2.

HTML

```<button id="differenceButton">Show difference</button>
<p id="differenceParagraph">Difference:</p>
```

JavaScript

```let differenceButton = document.getElementById(`differenceButton`)
let differenceParagraph = document.getElementById(`differenceParagraph`)

function showDifference() {
let difference = subtract(5, 3)
differenceParagraph.innerHTML = `Difference: \${difference}`
}

function subtract(// your code will appear here) {
return num1 - num2
}
```

Difference:

### Exercise 3 of 6

Write the return statement for the multiply function so that the result is 6.

HTML

```<button id="productButton">Show product</button>
<p id="productParagraph">Product:</p>
```

JavaScript

```let productButton = document.getElementById(`productButton`)
let productParagraph = document.getElementById(`productParagraph`)

function showProduct() {
let product = multiply(2, 3)
productParagraph.innerHTML = `Product: \${product}`
}

function multiply(num1, num2) {
// your code will appear here
}
```

Product:

### Exercise 4 of 6

Call the isPositive function.

HTML

```<button id="positiveButton">Show is positive</button>
<p id="positiveParagraph"></p>
```

JavaScript

```let positiveButton = document.getElementById(`positiveButton`)
let positiveParagraph = document.getElementById(`positiveParagraph`)

function showPositive() {
if (// your code will appear here) {
positiveParagraph.innerHTML = `The number is positive`
}
else {
positiveParagraph.innerHTML = `The number is not positive`
}
}

function isPositive(num) {
return num > 0
}
```

### Exercise 5 of 6

Write the parameters for the slice function so that the result is one, two, three.

HTML

```<button id="sliceButton">Show slice of array</button>
<p id="sliceParagraph">Slice of array:</p>
```

JavaScript

```let sliceButton = document.getElementById(`sliceButton`)
let sliceParagraph = document.getElementById(`sliceParagraph`)

function showSlice() {
let sliceOfArray = slice([`zero`, `one`, `two`, `three`, `four`], 1, 4)
sliceParagraph.innerHTML = `Slice of array: \${sliceOfArray.join(`, `)}`
}

// returns a portion of the array from a start index to an end index (end not included)
function slice(// your code will appear here) {
let resultArray = []

for (let i = start; i < end; i = i + 1) {
resultArray.push(array[i])
}

return resultArray
}
```

Slice of array:

### Exercise 6 of 6

Write the appropriate return statement for the includes function.

HTML

```<button id="includesButton">Show includes</button>
<p id="includesParagraph"></p>
```

JavaScript

```let includesButton = document.getElementById(`includesButton`)
let includesParagraph = document.getElementById(`includesParagraph`)

function showIncludes() {
if (includes([1, 1, 2, 3, 5], 3)) {
includesParagraph.innerHTML = `The array includes the value`
}
else {
includesParagraph.innerHTML = `The array does not include the value`
}
}

// returns true if the array contains the value, or false if it does not
function includes(array, valueToFind) {
for (let item in array) {
if (item == valueToFind) {
// your code will appear here
}
}

return false
}
```