Reduce a List
Run the reduce function on a list. Runs entirely in your browser, so your data never leaves your device.
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Output
The result appears here as you type.
How to use Reduce a List
- 1. Paste your list. Enter your items into the input pane, one per line. Any list you need collapsed down to a single computed result works here.
- 2. Write a reducer function. Type a JavaScript function body in the Reducer (acc, item, index) => result field. It runs once per item, combining the running accumulator with the current item into a new accumulator.
- 3. Set an initial value. Enter the starting value for the accumulator in Initial value. This is what the reducer receives as 'acc' before processing the very first item in the list.
- 4. Copy the final result. The output pane shows the single value left after the reducer has processed every item. Copy the result wherever the aggregated value is needed.
When to use Reduce a List
Reduce a List runs a custom accumulator function over every item in a list, folding it down to one final result the way a JavaScript reduce call does. Use it whenever you need to aggregate a list into a single computed value.
- Concatenating every item into one string. A list of characters or short fragments needs to be joined into a single continuous string, and writing 'acc + item' as the reducer accumulates them together.
- Summing a list of numeric strings. A list of numbers as text needs to be added together into a total, and writing a reducer that parses and adds each item produces that sum.
- Building a custom summary from list data. A list of records needs a specific aggregated summary that no built-in tool produces, and writing a reducer lets you define exactly how each item folds into the running result.
- Finding the longest item with a custom comparison. A list needs the single longest item found by comparing lengths as you go, and a reducer that keeps whichever of acc or item is longer returns that result directly.
Examples
Concatenate all items
Input
a b c
Output
abc
About the Reduce a List tool
Reduce a List does its work locally, right in the browser. Run the reduce function on a list. There is no upload step, no queue and no account, and your data never travels over the network.
It belongs to the List Tools collection on EditSafely, a set of 114 small, focused List utilities that share the same instant, private workspace.
You can shape the output with 2 settings, including Reducer (acc, item, index) => result and Initial value, and the result refreshes the moment you change one. A worked example further down the page shows exactly what the tool produces for a real input.
Running locally also makes the tool fast and dependable: results appear as you type or drop a file, there is no server outage that can take it down mid-task, and confidential data can be processed without a second thought.
Frequently asked questions
Does Reduce a List cost anything?
Yes, it is completely free. All 2,658 tools on EditSafely work without an account, a subscription or usage limits.
Is it safe to paste sensitive or confidential data?
No data leaves your device. The whole tool is JavaScript that runs inside your browser tab, so there is no upload, no server-side processing and no log of what you did. If you disconnect from the internet after the page loads, it keeps working.
How much text can I process at once?
There is no fixed limit. Because the work happens on your own device rather than on a shared server, the practical ceiling is your machine's memory, which comfortably handles inputs far larger than typical online tools allow.
Do I need to sign up or install anything?
Nothing to install and no account needed. Open the page in any up-to-date browser, including on a phone or tablet, and the tool is ready.
How do I use the result?
The output panel has a one-click copy button, and you can keep refining the input while you work; the result updates in place as you type.