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Use concat() or array spread to append elements; use object spread to combine the fields of two records; and use an explicit lookup when matching records by id. These operations have different effects: neither array operation merges matching objects, and every spread or concatenation shown here makes only a shallow copy.
Choose the operation that matches your goal
| Goal | Use | What it does |
|---|---|---|
| Append both arrays and keep duplicate elements | first.concat(second) or [...first, ...second] |
Returns 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 array without changing its inputs; array spread expands iterable elements into an array literal. |
| Combine properties of two individual objects | { ...first, ...second } |
Creates a shallow object copy; for a property present in both, the value from second wins. |
| Combine or deduplicate records with the same ID | Explicit lookup, such as a Map |
Requires a chosen rule for duplicate IDs, conflicting fields, ordering, and missing or malformed IDs. |
Append array elements with concat or spread
When the goal is simply to put every element into one array, either form works:
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const combined = first.concat(second);
// Or:
const combinedWithSpread = [...first, ...second];Both produce the first array’s elements followed by the second array’s elements, including any duplicate records. concat() creates a new shallow array and leaves the input arrays unchanged. Array spread also creates a new array literal by expanding the elements of each iterable; it does not inspect object IDs or combine object properties.
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Combine properties of two individual records
For two records that should become one object, object spread copies their enumerable own properties into a new object:
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const merged = { ...existing, ...incoming };Here, incoming takes precedence wherever both objects have the same property. To make the existing record win instead, reverse the order: { ...incoming, ...existing }. This is a top-level property merge, not a recursive merge of nested objects. The TypeScript 2.1 release notes document object spread for shallow copying and combining properties; that historical feature note is not a compatibility matrix for every current compiler target.
Merge arrays by ID with an explicit policy
Neither concat() nor array spread decides what counts as a matching record. The example below chooses one particular policy: retain the order in which IDs first appear, merge top-level fields for matching IDs, and let the second array win when fields conflict.
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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()];
}As the loop visits the first array and then the second, a new ID is inserted at its first occurrence. A later record with that ID replaces the map’s value without adding another output entry; spreading the previous record before the current one means the later record wins on conflicts. The returned values therefore appear in first-seen ID order.
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This is an illustrative policy, not a universal rule. If a later record should replace the entire earlier record, store { ...item } for every occurrence instead of spreading the previous value into it. If IDs can be absent, non-string, or case-insensitive, define and type that behavior explicitly rather than assuming the sample covers it.
Understand what stays shared
These operations are shallow. They create new outer arrays or objects, but do not recursively clone nested values. If a record contains a nested object or array, the copied record can still reference that same nested value. For nested properties, define whether sharing is acceptable or implement the required copying separately; do not assume array or object spread makes the result deeply independent. MDN describes these built-in copying operations as shallow in its documentation for concat(), spread syntax, and arrays.
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