Use Array.isArray(value) before calling map(). It confirms that a value is an actual array, unlike a broad typeof check or optional chaining.
Check the value before calling map()
Branch on Array.isArray() and decide what the non-array case should mean for your application:
if (Array.isArray(value)) {
return value.map(transform);
}
// Choose a fallback that matches the application's meaning.
return [];Array.isArray() returns true for arrays and false for values such as null, undefined, ordinary objects, strings, and typed arrays. It also recognizes arrays created in another JavaScript realm, such as an iframe. MDN’s Array.isArray() reference explains the check and its behavior.
Choose what to do when the value is not an array
The check prevents an invalid map() call, but the right response depends on why the value is missing or malformed. Returning an empty array is appropriate only if the application should treat every non-array as an empty collection. Otherwise, handle the unexpected value distinctly.
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- 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.
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Handle invalid or pending data separately
if (Array.isArray(items)) {
const labels = items.map((item) => item.label);
// use labels
} else {
// report invalid data, show a loading state, or handle it as appropriate
}This keeps an unexpected response from silently looking like a valid empty list.
Normalize when the data contract allows it
const safeItems = Array.isArray(items) ? items : [];
const labels = safeItems.map((item) => item.label);This concise pattern makes subsequent array operations safe, but it treats every non-array input as empty. Use it only when that is the intended meaning, rather than a way to conceal malformed data.
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- 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.)
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Check nested data at the point where you use it
Optional chaining and Array.isArray() solve different problems. Optional chaining lets a property chain pass through a missing or nullish value; the array predicate verifies the final value’s type.
const rows = Array.isArray(response?.data?.rows)
? response.data.rows.map(renderRow)
: [];Here, ?. protects access to nested properties, while Array.isArray() checks whether rows is an array. MDN’s optional chaining reference describes its nullish short-circuiting behavior.
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Why not use typeof, instanceof, or optional chaining alone?
typeof value === "object"is too broad. Arrays and many non-array values are objects, andtypeof nullis also"object". It does not establish thatmap()is appropriate.value instanceof Arrayis not reliable across realms. An array created in another realm may fail that check.Array.isArray()handles cross-realm arrays and rejects objects that merely inherit fromArray.prototype.value?.map(fn)is not an array check. It skips the access whenvalueisnullorundefined, but a present non-array value may still lack a callablemapmethod. Optional call syntax does not make an existing non-function callable.
For these distinctions, see MDN’s Array.isArray() reference and MDN’s optional chaining reference.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use map() when you need a transformed array
map() calls a callback for each present array element and builds a new array from the callback results. Keep the returned array when you need that transformation. If you only need to perform an action for each element and will discard the result, use forEach() or a for...of loop instead. See MDN’s Array.prototype.map() reference.
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
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- 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
For a value typed unknown in TypeScript
Use Array.isArray(value) as the runtime check before performing array operations on an unknown value. The element-type behavior depends on TypeScript’s type system and is not addressed here.
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- 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.
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