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Map vs forEach in Javascript

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What is the difference between the array methods map() and forEach()?

Answer

Both methods iterate through the elements of an array. map() maps each element to a new element by invoking the callback function on each element and returning a new array.

On the other hand, forEach() invokes the callback function for each element but does not return a new array. forEach() is generally used when causing a side effect on each iteration, whereas map() is a common functional programming technique.

Good to hear

  • Use forEach() if you need to iterate over an array and cause mutations to the elements without needing to return values to generate a new array.
  • map() is the right choice to keep data immutable where each value of the original array is mapped to a new array.

Code Examples

let arr = [1, 2, 3, 4, 5]

forEach()

arr.forEach((num, index) => {
  return (arr[index] = num * 2)
})
// arr = [2, 4, 6, 8, 10]

map()

let doubled = arr.map(num => {
  return num * 2
})
// doubled = [2, 4, 6, 8, 10]
// arr = [1, 2, 3, 4, 5] => arr is unchanged

Functional Programming

If you want to follow functional programming paradigm, map() is a better choice because forEach() will mutate the array elements directly, while map() will create a new array and keep the original array unchange.

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