Can you reverse an iteration step in JavaScript when using Array.prototype.map or Array.prototype.forEach? For instance, if the current index is 2, is it possible to go back to index 1 within these methods? While we can easily achieve this with a standard for-loop by decrementing i, is there a similar way to accomplish this using map?
To gain access to the element preceding it in the context of the .map method, you can utilize the third argument provided in the callback function:
const input = [1, 2, 3];
const output = input.map((num, i, arr) => {
console.log('last num was ' + arr[i - 1] + ', current num is ' + num);
return num + 1;
});However, it is not possible to directly modify or update the previous element within the newly mapped array since each iteration of the .map callback has already processed and returned a value for that specific index.
An alternative approach to achieve this functionality is by considering the elements following the current one rather than those preceding it, and use that information to determine the new mapped value:
const input = [1, 2, 3];
const output = input.map((num, i, arr) => {
console.log('next num is ' + arr[i + 1] + ', current num is ' + num);
return num + (arr[i + 1] || 0);
});
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