# find the missing number javascript

To find the missing number in JavaScript, there are a few different methods that can be used. The simplest and most efficient approach is to use a for loop. This loop will iterate through a given array and compare each element to the expected value based on its position in the array.

```
// Define a function to find the missing number
function findMissingNumber(arr) {
let expectedVal = 0;
let missingVal;
// Iterate over the array
for(let i=0; i
```

The `findMissingNumber()`

function takes an array as an argument and returns the missing value as an output. The loop works by setting an expected value for each element in the array (which should be equal to its index) and comparing it with the actual value. If the two values are not equal, the loop breaks and the missing value is stored in the `missingVal`

variable.

Another approach is to use the `reduce()`

method. This method will take a given array and reduce it to a single value. We can use this to calculate the sum of the expected values and compare it with the sum of the actual values. The difference between the two sums is the missing number.

```
// Define a function to find the missing number
function findMissingNumber(arr) {
// Calculate expected sum
let expectedSum = (arr.length + 1) * arr.length / 2;
// Calculate actual sum
let actualSum = arr.reduce((acc, curr) => acc + curr, 0);
// The difference is the missing number
return expectedSum - actualSum;
}
// Test with an example array
const exampleArr = [0, 1, 2, 3, 5];
console.log(findMissingNumber(exampleArr)); // Output: 4
```

The `findMissingNumber()`

function takes an array as an argument and returns the missing value as an output. The expected sum is calculated using the formula `(n + 1) * n / 2`

, where `n`

is the length of the array, and the actual sum is calculated using the `reduce()`

method. The difference between the two sums is then returned as the missing value.

These two methods are the most efficient ways to find the missing number in JavaScript. While they may not be the only approaches, they are both relatively straightforward and can be used to quickly identify and return the missing value in an array.