I'm transforming my array with Array.prototype.map(), and if the current item matches some conditions, I want to change its previous item.
The code below seems not working, how do I do it without a for loop?
var nums = [1, 2, 3, 4, 5];
var res = nums.map((item, index, array) => {
if (item > 3) {
array[index - 1] += 1;
}
return item;
});
console.log(res); // [ 1, 2, 3, 4, 5 ], I want [1, 2, 4, 5, 5]You can use a reducer
var res = [1, 2, 3, 4, 5]
.reduce((reduced, item, i) => {
i && (reduced[i-1] += item > 3 ? 1 : 0);
return [...reduced, item];
}, []);
console.log(res); Arrays that are in the process of being created from .map can't be referenced like that. Instead, create the array outside of the forEach, and push to / assign to it:
var nums = [1, 2, 3, 4, 5];
const newArr = [];
nums.forEach((item, index) => {
if (item > 3) newArr[index - 1] += 1;
newArr.push(item);
});
console.log(newArr); // I want [1, 2, 4, 5, 5]
.map() is what's called an out-of-place algorithm, which means that the output array is not the same reference as the input array.Disclaimer: this answer is only here because I feel whenever somebody asks "how do I do X without a for loop", I feel there should at least be one recursive solution between all the maps and reduces :)
how do I do it without a for loop?
If you don't care much about performance or the risk of blowing the stack, you can go with a recursive function!
Here's an example:
first, second and the restfirst is empty, we've reached the end of our computation and return the resultfirst is not empty, we can compare it to second to determine its value. We add the value to our result and recurse:[second, ...rest] and passing along our resultconst input = [ 1, 2, 3, 4, 5 ];
const Empty = Symbol();
const incIfBt3 =
([first = Empty, second, ...rest], result = []) =>
first === Empty
? result
: incIfBt3(
[second, ...rest],
[...result, first + (second > 3)]
);
console.log(incIfBt3(input));
You can look ahead instead of trying to look behind:
var nums = [1, 2, 3, 4, 5];
var res = nums.map((item, index, array) => {
if (array[index + 1] > 3) {
item += 1;
}
return item;
});
Instead of trying to change the previous item, you can use the array parameter to look at the next item and return a value for the current item accordingly:
var nums = [1, 2, 3, 4, 5];
var res = nums.map((item, index, array) => item + (array[index + 1] > 3 ? 1 : 0));
console.log(res); // [1, 2, 4, 5, 5]You can also take advantage of the fact that the numeric value of a boolean is either 1 or 0 to eliminate the conditional expression:
var nums = [1, 2, 3, 4, 5];
var res = nums.map((item, index, array) => item + (array[index + 1] > 3));
console.log(res); // [1, 2, 4, 5, 5]