Sure, here is a step by step solution in JavaScript:

1. First, define the function that will transform the array. This function will take two parameters: the array to be transformed and the mapping function.

```javascript
function transformArray(arr, fn) {
```

2. Inside this function, initialize an empty array to store the transformed elements.

```javascript
var transformedArray = [];
```

3. Then, use a for loop to iterate over each element in the input array.

```javascript
for (var i = 0; i

Question

Sure, here is a step by step solution in JavaScript:

1. First, define the function that will transform the array. This function will take two parameters: the array to be transformed and the mapping function.

```javascript
function transformArray(arr, fn) {
```

2. Inside this function, initialize an empty array to store the transformed elements.

```javascript
    var transformedArray = [];
```

3. Then, use a for loop to iterate over each element in the input array.

```javascript
    for (var i = 0; i < arr.length; i++) {
```

4. Inside the loop, apply the mapping function to the current element and its index, and store the result in the transformed array.

```javascript
        transformedArray[i] = fn(arr[i], i);
```

5. Close the for loop.

```javascript
    }
```

6. After the loop, return the transformed array.

```javascript
    return transformedArray;
```

7. Finally, close the function definition.

```javascript
}
```

So, the complete function looks like this:

```javascript
function transformArray(arr, fn) {
    var transformedArray = [];
    for (var i = 0; i < arr.length; i++) {
        transformedArray[i] = fn(arr[i], i);
    }
    return transformedArray;
}
```

You can use this function by passing an array and a mapping function as arguments. For example:

```javascript
var arr = [1, 2, 3, 4, 5];
var fn = function(x, i) { return x * i; };
console.log(transformArray(arr, fn));  // Outputs: [0, 2, 6, 12, 20]
```

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