Thirty functions total!
This is my third article on Utility Functions Made with Reduce.
Altogether we now have thirty helper functions made using JavaScript's reduce. For more detail on reduce itself, consider reading my tutorial on it.
The functions listed below were inspired by the amazing RamdaJS library. I also wrote unit tests to ensure correct behavior, which you can find on my GitHub.
1. adjust
Parameters
index- Index of source arrayfn- Function to apply at thatindexlist- List of items.
Description
Applies a function to the value at the given index. Returns the original array if provided index is out of bounds.
Usage
const double = (x) => x * 2;
adjust(1, double, [1, 2, 3]);
// [1, 4, 3]
adjust(4, double, [1, 2, 3]);
// [1, 2, 3]
Implementation
const adjust = (index, fn, list) =>
list.reduce((acc, value, sourceArrayIndex) => {
const valueToUse = sourceArrayIndex === index ? fn(value) : value;
acc.push(valueToUse);
return acc;
}, []);
2. fromPairs
Parameters
pairs- A list of key-value pairs.
Description
Creates an object from a list of key-value pairs.
Usage
fromPairs([['a', 1], ['b', 2], ['c', 3]]);
// { a: 1, b: 2, c: 3 }
Implementation
const fromPairs = (pairs) =>
pairs.reduce((acc, currentPair) => {
const [key, value] = currentPair;
acc[key] = value;
return acc;
}, {});
3. range
Parameters
from- Starting number.to- Ending number.
Description
Returns a list of numbers from a given range.
Usage
range(1, 5);
// [1, 2, 3, 4, 5]
Implementation
const range = (from, to) =>
Array.from({ length: to - from + 1 }).reduce((acc, _, index) => {
acc.push(from + index);
return acc;
}, []);
4. repeat
Parameters
item- Item to repeat.times- Number of times to repeat.
Description
Returns a list of a given value N times.
Usage
repeat({ favoriteLanguage: 'JavaScript' }, 2);
/*
[{
favoriteLanguage: 'JavaScript'
}, {
favoriteLanguage: 'JavaScript'
}],
*/
Implementation
const repeat = (item, times) =>
Array.from({ length: times }).reduce((acc) => {
acc.push(item);
return acc;
}, []);
5. times
Parameters
fn- Function to call.numTimes- Number of times to call that function.
Description
Calls a given function N times. The fn provided receives the current index as a parameter.
Usage
times((x) => x * 2, 3);
// [0, 2, 4]
Implementation
const times = (fn, numTimes) =>
Array.from({ length: numTimes }).reduce((acc, _, index) => {
acc.push(fn(index));
return acc;
}, []);
6. deduplicate
Parameters
items- List of items.
Description
Deduplicates the items in a list.
Usage
deduplicate([[1], [1], { hello: 'world' }, { hello: 'world' }]);
// [[1], { hello: 'world' }]
Implementation
const deduplicate = (items) => {
const cache = {};
return items.reduce((acc, item) => {
const alreadyIncluded = cache[item] === true;
if (!alreadyIncluded) {
cache[item] = true;
acc.push(item);
}
return acc;
}, []);
};
7. reverse
Parameters
list- List of items.
Description
Reverses a list without mutating it by returning a new list, unlike the native Array.reverse method.
Usage
reverse([1, 2, 3]);
// [3, 2, 1]
Implementation
const reverse = (list) =>
list.reduce((acc, _, index) => {
const reverseIndex = list.length - index - 1;
const reverseValue = list[reverseIndex];
acc.push(reverseValue);
return acc;
}, []);
8. insertAll
Parameters
index- Index to insert at.subList- List to insert.sourceList- Source list.
Description
Inserts a sub-list into a list at the given index. Appends to the end of the list if index is too large.
Usage
insertAll(1, [2, 3, 4], [1, 5]);
// [1, 2, 3, 4, 5]
insertAll(10, [2, 3, 4], [1, 5]);
// [1, 5, 2, 3, 4]
Implementation
const insertAll = (index, subList, sourceList) => {
if (index > sourceList.length - 1) {
return sourceList.concat(subList);
}
return sourceList.reduce((acc, value, sourceArrayIndex) => {
if (index === sourceArrayIndex) {
acc.push(...subList, value);
} else {
acc.push(value);
}
return acc;
}, []);
};
9. mergeAll
Parameters
objectList- List of objects to merge.
Description
Merges a list of objects into one.
Usage
mergeAll([
{ js: 'reduce' },
{ elm: 'fold' },
{ java: 'collect' },
{ js: 'reduce' }
]);
/*
{
js: 'reduce',
elm: 'fold',
java: 'collect'
}
*/
Implementation
const mergeAll = (objectList) =>
objectList.reduce((acc, obj) => {
Object.keys(obj).forEach((key) => {
acc[key] = obj[key];
});
return acc;
}, {});
10. xprod
Parameters
list1- List of items.list2- List of items.
Description
Given a list, returns a new list of all possible pairs.
Usage
xprod(['Hello', 'World'], ['JavaScript', 'Reduce']);
/*
[
['Hello', 'JavaScript'],
['Hello', 'Reduce'],
['World', 'JavaScript'],
['World', 'Reduce']
]
*/
Implementation
const xprod = (list1, list2) =>
list1.reduce((acc, list1Item) => {
list2.forEach((list2Item) => {
acc.push([list1Item, list2Item]);
});
return acc;
}, []);
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