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const numbers = [1, 4, 9];
const roots = numbers.map((num) => Math.sqrt(num));
// roots is now [1, 2, 3]
// numbers is still [1, 4, 9]
The map()
method is an iterative method. It calls a provided callbackFn
function once for each element in an array and constructs a new array from the results.
callbackFn
is invoked only for array indexes which have assigned values. It is not invoked for empty slots in sparse arrays.
The map()
method is a copying method. It does not alter this
. However, the function provided as callbackFn
can mutate the array. Note, however, that the length of the array is saved before the first invocation of callbackFn
. Therefore:
callbackFn
will not visit any elements added beyond the array's initial length when the call to map()
began.
Changes to already-visited indexes do not cause callbackFn
to be invoked on them again.
If an existing, yet-unvisited element of the array is changed by callbackFn
, its value passed to the callbackFn
will be the value at the time that element gets visited. Deleted elements are not visited.
const numbers = [1, 4, 9];
const roots = numbers.map((num) => Math.sqrt(num));
// roots is now [1, 2, 3]
// numbers is still [1, 4, 9]
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