iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
Use concat() or array spread to append array elements; use object spread to combine the fields of two objects; and use explicit lookup logic to merge array records by id. These operations have different results: neither concat() nor array spread automatically matches records, and all of these copies are shallow.
Choose the operation that matches your goal
| Goal | Use | What it does |
|---|---|---|
| Append every element and retain duplicates | first.concat(second) or [...first, ...second] |
Creates an array with the elements of first followed by those of second; it does not merge object fields. MDN documents that concat() returns a new shallow array, while MDN describes array spread as expanding iterable elements. |
| Combine two known objects | { ...first, ...second } |
Copies enumerable own properties into a new object; for overlapping keys, the later object’s value wins. TypeScript’s 2.1 release notes describe object spread for combining objects. |
| Combine or deduplicate records by ID | Explicit lookup logic, such as a Map |
You choose how duplicate IDs, conflicting fields, ordering, and missing IDs should be handled. Neither array operation makes those decisions for you. |
Append arrays without merging their objects
If you want all elements from both arrays, including records with repeated IDs, either form is suitable:
const combined = first.concat(second);
// Or:
const combined = [...first, ...second];Both put the elements of first before the elements of second. concat() returns a new array and does not mutate its input arrays. Array spread also creates an array from the elements of its iterables. Neither operation inspects object IDs or combines object properties. See MDN’s concat() reference and spread syntax reference.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Combine the properties of two objects
For two individual records that should become one object, object spread provides a shallow property merge:
#1 Best Overall
- Read Before You Buy — No Video Output: These adapters support charging and USB 2.0 data transfer, but cannot transmit video signals. Except for standard USB webcams (which use USB data only), they are not compatible with HDMI/DisplayPort cables, video-capable USB-C hubs, or docking stations with video output.
- Convert USB-A Ports to USB-C: Designed to connect USB-C earphones, cables, flash drives, card readers, and other USB-C accessories to standard USB-A ports. Plug-and-play with no drivers or software required.
- Aluminum Alloy Housing: Built with a sturdy aluminum alloy shell that aids in heat dissipation and protects against daily wear and scratches. Designed to maintain a stable and secure connection.
- Compact & Travel-Friendly: The ultra-compact design allows the adapter to stay plugged into your device without blocking adjacent ports or adding bulk, reducing wear and tear on your original USB ports.
- 12-Month Warranty: Backed by a 12-month manufacturer warranty for peace of mind. Designed to meet strict quality control standards for reliable everyday performance.
const merged = { ...existing, ...incoming };When both objects have the same key, the value from incoming wins because it appears later. To let existing win instead, reverse the order: { ...incoming, ...existing }. This is a top-level merge, not a recursive merge of nested objects. The MDN spread syntax reference explains the distinct array and object forms; the TypeScript 2.1 release notes document the historical introduction of object spread support, not a compatibility matrix for every current compiler target.
Merge array records by ID
A by-ID merge needs a policy. The example below keeps the order in which IDs first appear, combines matching records’ top-level properties, and lets the second array win for conflicting fields:
Rank #2
- 5-in-1 USB-C Hub: Experience comprehensive connectivity featuring a Power Delivery input, two USB-A 2.0 ports, a USB-A 3.0 port, and an HDMI port. (Note: The USB-C power delivery input port is only for connecting an external wall charger to power your laptop and cannot power peripheral devices.)
- 90W Pass-Through Charging: Achieve optimal charging with 90W pass-through power to your laptop, supported by a total input of 100W, with the hub reserving 10W for operational efficiency. (Note: Wall charger not included.)
- Quick Data Transfers: Accelerate your productivity with rapid data transfers using a high-speed 5Gbps USB 3.0 port and two 480Mbps USB 2.0 ports.
- 4K HDMI Display: Enhance your visual experience with a hub capable of delivering 4K resolution at 30Hz in both mirror and extend modes. Please note that this hub is compatible with MacBook (macOS 12 and newer), Windows 10 and 11, ChromeOS, and laptops equipped with DP Alt Mode and Power Delivery. Note: This device is not compatible with Linux.
- What You Get: Anker USB-C Hub (5-in-1, 4K HDMI), welcome guide, 18-month warranty, and our friendly customer service.
type Item = { id: string; [key: string]: unknown };
function mergeById(first: Item[], second: Item[]): Item[] {
const byId = new Map<string, Item>();
for (const item of [...first, ...second]) {
const previous = byId.get(item.id);
byId.set(item.id, previous ? { ...previous, ...item } : { ...item });
}
return [...byId.values()];
}The loop processes the first array and then the second. The first appearance of an ID determines its position in the result; when that ID appears again, the stored record is updated rather than added as another entry. Object spread combines only top-level enumerable properties, with the later record taking precedence. The TypeScript and MDN references above describe the underlying spread behavior; this particular Map-based policy is an illustrative implementation, not a prescribed rule.
Free tools Windows power users keep installed
One-click scans. No signup required.
Adapt the policy to your data
- If a later record should replace the earlier record entirely, store
itemfor an existing ID instead of spreading both records together. - If an ID may be missing, nullable, or not a string, represent that possibility in the type and decide whether to skip, reject, or handle such records separately.
- If IDs are case-insensitive, normalize them consistently before using them as keys.
- If duplicate records should remain distinct, do not deduplicate them with a map; use concatenation instead.
Account for shallow copies
concat(), array spread, and object spread copy only one level. The result array is new, but its object elements may still be the same objects held in the input arrays. Likewise, a newly spread object can retain references to nested objects or arrays from its source. Changing a nested value can therefore affect data reachable through an input as well as through the result. These operations do not provide deep independence; see MDN on shallow concatenation, MDN on spread syntax, and MDN’s Array reference.
Rank #3
- Sleek 7-in-1 USB-C Hub: Features an HDMI port, two USB-A 3.0 ports, and a USB-C data port, each providing 5Gbps transfer speeds. It also includes a USB-C PD input port for charging up to 100W and dual SD and TF card slots, all in a compact design.
- Flawless 4K@60Hz Video with HDMI: Delivers exceptional clarity and smoothness with its 4K@60Hz HDMI port, making it ideal for high-definition presentations and entertainment. (Note: Only the HDMI port supports video projection; the USB-C port is for data transfer only.)
- Double Up on Efficiency: The two USB-A 3.0 ports and a USB-C port support a fast 5Gbps data rate, significantly boosting your transfer speeds and improving productivity.
- Fast and Reliable 85W Charging: Offers high-capacity, speedy charging for laptops up to 85W, so you spend less time tethered to an outlet and more time being productive.
- What You Get: Anker USB-C Hub (7-in-1), welcome guide, 18-month warranty, and our friendly customer service.
What TypeScript does—and does not—decide
TypeScript checks types at compile time, but it does not choose your duplicate-ID policy, field precedence, result ordering, or deep-copy behavior. Define those rules in the code and types that fit your application’s data. For other array transformations, MDN’s Array reference covers methods including map(), filter(), and reduce(); select a method based on whether you are transforming, selecting, or aggregating elements.
Quick Recap
Best Value
- 5-in-1 Connectivity: Equipped with a 4K HDMI port, a 5 Gbps USB-C data port, two 5 Gbps USB-A ports, and a USB C 100W PD-IN port. Note: The USB C 100W PD-IN port supports only charging and does not support data transfer devices such as headphones or speakers.
- Powerful Pass-Through Charging: Supports up to 85W pass-through charging so you can power up your laptop while you use the hub. Note: Pass-through charging requires a charger (not included). Note: To achieve full power for iPad, we recommend using a 45W wall charger.
- Transfer Files in Seconds: Move files to and from your laptop at speeds of up to 5 Gbps via the USB-C and USB-A data ports. Note: The USB C 5Gbps Data port does not support video output.
- HD Display: Connect to the HDMI port to stream or mirror content to an external monitor in resolutions of up to 4K@30Hz. Note: The USB-C ports do not support video output.
- What You Get: Anker 332 USB-C Hub (5-in-1), welcome guide, our worry-free 18-month warranty, and friendly customer service.
Rank #4
- Dual Converters, Infinite Potential：Includes 2× USB C male to USB A female adapters and 2× USB A male to USB C female adapters. Perfect for a wide range of uses—tablets with Bluetooth keyboards, expand USB ports on macbook, and more. Two different converters for all your daily needs
- Next-Level 10Gbps & 3A Charging: No more slow 480Mbps, this usb to usb c adapter has a transfer speed of up to 10Gbps, allowing you to do more transferring in less time. This usb adapter fits both USB A and USB C charger, supporting up to 3A fast charging
- Upgraded Exquisite Craftsmanship: With an aluminum alloy housing and metal connector, the usbc to usb adapter is extremely durable and sturdy. Rigorously tested to withstand more than 10,000 times of plugging and unplugging, ensuring long-lasting performance
- Broad Compatible: The usb c to usb adapter widely supports all USB C/ USB A devices like laptops, tablets, cellphones, car chargers, and phone chargers. Such as compatible with MacBook Pro/Air 2023/2022, Thunderbolt 4/3 Devices,Apple MagSafe Watch 9/8/7/SE/Ultra, iPad Pro 2022/2021, Samsung Galaxy S23/S20/S10, and iPhone 17/16/15 Pro. Plug and play
- Please Note: To reach 10Gbps speed, keep the cable under 3.3 ft. For USB A Male to USB C adapters, try flipping the USB C connector. USB C Male to USB A adapters support bidirectional 10Gbps transfer within 3.3 ft
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.
