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Use arr.reverse() to reverse an array in place. To get the reversed order without changing the original, use arr.toReversed() when your runtime supports it, or reverse a shallow copy with [...arr].reverse().
Reverse an array in place with reverse()
reverse() changes the array it is called on and returns that same array reference:
const values = [1, 2, 3];
const result = values.reverse();
console.log(values); // [3, 2, 1]
console.log(result === values); // trueBecause the original array changes, every variable or object property that refers to it sees the reversed order. Assigning the return value to a different variable does not preserve the previous order.
Keep the original order unchanged
Use toReversed() when supported
toReversed() creates and returns a new array, leaving the original array’s order unchanged:
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const values = [1, 2, 3];
const reversed = values.toReversed();
console.log(reversed); // [3, 2, 1]
console.log(values); // [1, 2, 3]TypeScript includes the method’s declaration in its ES2023 library APIs, as documented in the TSConfig lib reference. However, TypeScript declarations only tell the compiler which APIs to recognize; they do not add the method to the JavaScript runtime. Check that the runtime where your code executes supports toReversed(), or use a suitable polyfill or the copy-first alternative below. Since a project’s target and lib settings can differ, a successful type check alone does not prove runtime support.
Use a copy-first fallback
For runtimes without toReversed(), copy the array and then reverse the copy:
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const values = [1, 2, 3];
const reversed = [...values].reverse();
// Equivalent: values.slice().reverse()Both expressions leave the original array’s order unchanged. They create a shallow copy, not a deep copy: if an element is an object, the original and copied arrays still refer to that same object. See MDN’s explanation of Array reference and copying.
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- To change the existing mutable array, call
arr.reverse(). - To return a new reversed array on a runtime that supports it, call
arr.toReversed(). - To return a new reversed array without relying on
toReversed(), use[...arr].reverse()orarr.slice().reverse().
A variable typed as readonly T[] cannot be mutated through that readonly type. TypeScript describes ReadonlyArray as a type for arrays that should not be changed in its Object Types handbook. Use a copy-first expression for a mutable reversed result, or toReversed() when both the library declarations and runtime support it.
How sparse arrays behave
For ordinary dense arrays, the methods produce the expected reversed element order. Sparse arrays have a distinction: reverse() preserves empty slots in their corresponding reversed positions, while toReversed() reads holes as undefined and returns a dense array at those positions. Consult MDN’s references for reverse() and toReversed() if your code relies on sparse-array behavior.
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