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This approach answers common questions such as “How do I filter scraped results?”, “How do I remove duplicates from scraped data?”, “Should I use map, filter, or reduce?”, and “How do I edit an array without changing the original?” The examples below use plain JavaScript, so you can adapt them to browser automation, a server-side scraper, or data received from an API.
Model scraped output as records
Start by converting parser output into predictable objects. A record might contain title, url, price, and available. Consistent field names and value types make every later stage easier to test.
const raw = [
{ title: " Alpha ", href: "/a", priceText: "$12" },
{ title: "", href: "/missing", priceText: "" },
{ title: "Beta", href: "/b", priceText: "$9" }
];JavaScript arrays are zero-based: the first element is at index 0. Scrapers also commonly produce missing fields or empty slots. Normalize those cases explicitly rather than relying on sparse-array behavior.
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Normalize every row with map()
map() performs a one-to-one transformation and returns a new array. MDN defines it as creating “a new array populated with the results of calling a provided function on every element in the calling array.” Use it to trim text, resolve relative links, parse numbers, and rename fields.
const normalized = raw.map((item) => ({
title: (item.title ?? "").trim(),
url: new URL(item.href, "https://example.com").href,
price: Number((item.priceText ?? "").replace(/[^0-9.]/g, ""))
}));
console.log(normalized);
// [
// { title: "Alpha", url: "https://example.com/a", price: 12 },
// { title: "", url: "https://example.com/missing", price: 0 },
// { title: "Beta", url: "https://example.com/b", price: 9 }
// ]Always use the returned array. Calling map() only for side effects is an anti-pattern; use forEach() or for...of when you do not need a transformed array.
Keep valid rows with filter()
filter() also returns a new array, retaining elements for which its predicate returns a truthy value. Put quality rules in one place: require a title, a finite price, and (when appropriate) an allowed host.
const records = normalized.filter((item) => {
let hostIsAllowed = false;
try {
hostIsAllowed = new URL(item.url).hostname === "example.com";
} catch {
hostIsAllowed = false;
}
return item.title.length > 0 &&
Number.isFinite(item.price) &&
hostIsAllowed;
});To remove duplicate scraped records, filter against a key while maintaining a Set of URLs:
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const unique = records.filter((item) => {
if (seen.has(item.url)) return false;
seen.add(item.url);
return true;
});This keeps the first occurrence. If you need the last occurrence, iterate from the end or build a Map keyed by URL and then read its values.
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Choose map(), filter(), or reduce()
| Method | Purpose | Returns | Mutates source? | Typical scraping use |
|---|---|---|---|---|
map() |
One-to-one transformation | New array | No | Rename fields, parse prices, resolve URLs |
filter() |
Predicate-based selection | New array | No | Drop empty titles, invalid prices, unwanted hosts |
reduce() |
Accumulation into one result | Any value | Not unless you mutate the accumulator | Totals, grouping, counts, indexes |
slice() |
Non-destructive range | New array | No | Pagination or previews |
splice() |
Positional insertion, replacement, deletion | Removed elements | Yes | Intentional in-place edits |
toSpliced() |
Non-mutating splice equivalent | New array | No | Edits while preserving the source |
Aggregate with reduce()
Use reduce() when the result is not another row-for-row array. For example, calculate a price total:
const total = records.reduce(
(sum, item) => sum + item.price,
0
);Group records by a field:
const byHost = records.reduce((groups, item) => {
const host = new URL(item.url).hostname;
(groups[host] ??= []).push(item);
return groups;
}, {});Or create a URL index for fast lookup:
const byUrl = records.reduce((index, item) => {
index[item.url] = item;
return index;
}, {});Choose an initial accumulator such as 0, {}, or []. An explicit initial value avoids errors on an empty scrape and documents the result type.
Paginate and edit without losing the source
Read a page with slice()
slice(start, end) returns a shallow copy and leaves the source untouched. For 20-item pages:
const pageSize = 20;
const pageNumber = 2;
const page = records.slice(
(pageNumber - 1) * pageSize,
pageNumber * pageSize
);Delete or replace in place with splice()
splice() changes the array itself. Before deleting by value, find the index and guard against -1:
const index = records.findIndex((item) => item.url === "https://example.com/a");
if (index !== -1) records.splice(index, 1);Use splice(index, 1, replacement) to replace one item, or splice(index, 0, item) to insert. Because later pipeline stages will see the changed array, reserve this for deliberate working-copy edits.
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Prefer toSpliced() for immutable edits
Where your runtime supports it, toSpliced() performs the same positional operation without changing the original:
const withoutFirst = records.toSpliced(0, 1);
const corrected = records.toSpliced(1, 1, {
...records[1],
title: "Corrected title"
});For older runtimes, combine slice() and spread syntax, or make a shallow copy before calling splice(). Remember that these are shallow copies: nested objects remain shared, so clone nested data separately when you will mutate it.
A complete scraping-array pipeline
const raw = [
{ title: " Alpha ", href: "/a", priceText: "$12" },
{ title: "", href: "/missing", priceText: "" },
{ title: "Beta", href: "/b", priceText: "$9" },
{ title: "Alpha duplicate", href: "/a", priceText: "$12" }
];
const records = raw
.map((item) => ({
title: (item.title ?? "").trim(),
url: new URL(item.href, "https://example.com").href,
price: Number((item.priceText ?? "").replace(/[^0-9.]/g, ""))
}))
.filter((item) => item.title && Number.isFinite(item.price));
const seen = new Set();
const unique = records.filter((item) => {
if (seen.has(item.url)) return false;
seen.add(item.url);
return true;
});
const totals = unique.reduce(
(sum, item) => sum + item.price,
0
);
const firstPage = unique.slice(0, 20);
const workingCopy = unique.toSpliced(0, 1);
console.log({ unique, totals, firstPage, workingCopy });Each stage has one job, which makes failures easy to locate: inspect the raw parser output, then the normalized values, then validation, deduplication, aggregation, and export.
Export and hand off the result
After shaping the array, serialize it for the next stage:
const json = JSON.stringify(unique, null, 2);
// In Node.js: await fs.promises.writeFile("results.json", json);
const csvRows = [
["title", "url", "price"],
...unique.map((item) => [item.title, item.url, item.price])
];
const csv = csvRows
.map((row) => row.map((value) => `"${String(value).replaceAll('"', '""')}"`).join(","))
.join("n");For large crawls, process pages incrementally instead of retaining every record. Persist checkpoints, and make your deduplication key explicit (canonical URL, product ID, or another stable identifier).
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Common errors and fixes
NaNprices: currency text may contain commas, symbols, or localized decimals. Normalize according to the source locale and reject non-finite results.- Relative URLs: pass the href and a known base URL to
new URL(); catch malformed values before filtering. - Unexpected source changes: selectors can return
null. Use optional chaining or nullish coalescing before callingtrim(). - Everything disappears after filtering: log each predicate separately and verify that host, title, and numeric rules match the site’s actual output.
- Duplicate pages: canonicalize URLs (for example, consistently remove or retain trailing slashes and tracking parameters) before using a
Set. - Original data changed unexpectedly: search for mutating methods such as
push(),pop(),shift(),unshift(),reverse(), andsplice(). Useslice()ortoSpliced()for non-mutating alternatives. - Empty-array reduction error: supply an initial accumulator to
reduce().
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FAQ
How do I remove duplicates when no single field is unique?
Build a deterministic composite key, such as a normalized URL plus product ID, and store that key in a Set before accepting the record.
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Does filter() modify objects inside the array?
No. It creates a new array, but both arrays still reference the same object values. Clone an object before changing its properties.
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When should I use a loop instead of chained methods?
Use a loop when one pass materially improves memory or performance, when you need early termination, or when several side effects must be coordinated. Keep separate stages when clarity and auditability matter more.
Frequently Asked Questions
How do I remove duplicates when no single field is unique?
Build a deterministic composite key, such as a normalized URL plus product ID, and store that key in a Set before accepting the record.
Does filter() modify objects inside the array?
No. It creates a new array, but both arrays still reference the same object values. Clone an object before changing its properties.
When should I use a loop instead of chained methods?
Use a loop when one pass materially improves memory or performance, when you need early termination, or when several side effects must be coordinated.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
