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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsUse array.push(value) to append to a mutable TypeScript array; it changes that array and returns its new length. To add an item at the beginning, use unshift. To preserve the original array, create a new one with spread syntax or concat. If your input is typed as readonly, build and return a new array rather than trying to call a mutating method.
Append elements to the end with push
TypeScript arrays use JavaScript array methods. For an array you are allowed to change, call push with the item to append:
const names: string[] = ["Ada", "Lin"];
names.push("Grace");
// names is now ["Ada", "Lin", "Grace"]The TypeScript Handbook gives the method signature as push(...items: Type[]): number: it accepts one or more values of the array’s element type and returns the new length, not a replacement array. See the TypeScript Handbook’s array-method documentation.
const newLength = names.push("Edsger");To append several values, pass them as separate arguments. To append the contents of another array, spread that array into the call:
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names.push("Barbara", "Katherine");
const moreNames: string[] = ["Alan", "Grace"];
names.push(...moreNames);For a spread argument in a function call, the argument types must fit the function’s parameters. In cases involving a fixed set of arguments, TypeScript may need the spread value to be a tuple; as const can preserve a literal array as a tuple. Check the Handbook’s spread-argument and tuple guidance when the compiler rejects a spread call.
Add an element at the beginning with unshift
Use unshift when new elements belong before the current first element. It mutates the array, placing the supplied values first and moving the existing elements after them:
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const names = ["Ada", "Lin"];
names.unshift("Grace");
// names is now ["Grace", "Ada", "Lin"]For the documented behavior of unshift and array-combination methods, see MDN’s array-method reference.
Add elements without changing the original array
Use array spread to create a new array when the original should remain intact. Put the new value after the spread to append it, or before the spread to prepend it:
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const original = [1, 2, 3];
const withFour = [...original, 4];
const withZero = [0, ...original];original remains [1, 2, 3]; the new arrays contain the added values. You can also use concat to create a combined array:
const alsoWithFour = original.concat(4);
const combined = original.concat([5, 6]);For two arrays, spread and concat both create a combined result:
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const first = [1, 2];
const second = [3, 4];
const combinedWithSpread = [...first, ...second];
const combinedWithConcat = first.concat(second);Choose the method that matches the operation
| Need | Approach | Effect |
|---|---|---|
| Append to a mutable array | array.push(value) |
Changes the existing array; returns its new length. |
| Insert at the beginning | array.unshift(value) |
Changes the existing array by adding at the front. |
| Return an appended copy | [...array, value] |
Creates a new array and leaves the source array intact. |
| Combine arrays into a new result | [...first, ...second] or first.concat(second) |
Produces a combined array. TypeScript 4.0 documents typed variadic tuple concatenation in its release notes. |
| Accept input callers can share without permitting mutation through that reference | readonly T[] or ReadonlyArray<T> |
Communicates read-only use in TypeScript and omits mutating methods from that typed reference. |
Work with readonly arrays
A function can accept a readonly array when it only needs to read the input. To add an item, return a fresh mutable array:
function withItem<T>(items: readonly T[], item: T): T[] {
return [...items, item];
}Readonly array types do not provide mutating methods such as push. TypeScript also does not let you assign a readonly array to a mutable array variable without an assertion. Prefer creating a copy, as above, instead of asserting away the restriction. The TypeScript Handbook explains readonly arrays and describes ReadonlyArray as “a special type that describes arrays that shouldn’t be changed.”
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readonly is a TypeScript compile-time constraint, not a runtime freeze. Another reference to the same JavaScript array can still change it. If you need runtime immutability, it requires a separate runtime design.
Keep tuple types when positions are significant
Ordinary arrays describe a sequence of elements of a type; tuples describe fixed positions and their types. When combining tuples, preserving their positional information can matter. TypeScript 4.0 introduced variadic tuple types, which support typed tuple concatenation; use as const when a literal array needs to be inferred as a fixed tuple. The TypeScript 4.0 release notes show the tuple-concatenation capability.
Check configuration when spread syntax fails
The TypeScript Handbook notes that spread arguments may require tuple types or rest parameters to match a function’s parameters. For projects targeting older runtimes, support for downlevel iteration may also depend on the project’s compiler target and downlevelIteration setting. Check the project’s TypeScript configuration and target rather than assuming every runtime handles iteration the same way; the functions documentation covers spread arguments and this compatibility consideration.
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