For Each / Map

forEach(func(value, index, array))

• forEach() will iterate over each element of an array and apply a provided function to each element as it iterates. The function will be passed three arguments:
  1. value: this will be the value at the current index of the array.
  2. index: will be the numerical index of the current element.
  3. array: the array currently being iterated over (the entire array)
• The forEach() function is non-destructive, but you can alter values during iteration by using the arguments passed to your function.

let nums = [1,2,3,4,5];

// iterate over an array, log the square of each value
nums.forEach(function(value, index, array){
  console.log(value * value);
});

// results:
1
4
9
16
25

nums; // [ 1, 2, 3, 4, 5 ] => non-destructive

// iterate over an array, square the value in each position
nums.forEach(function(value, index, array){
    array[index] = value * value;
});

nums; // [ 1, 4, 9, 16, 25 ] => updated values

map(func(currentValue, idx, arr))

• Similar to forEach(), map() iterates over an array, taking an anonymous function as an argument, which will perform an operation on each element of the array. The returned value of the function will become the value of the element.

map returns an array containing the results of performing the function on each element.

map is also passed three values:

1. currentValue: this will be the value at the current index of the array.
2. idx: will be the numerical index of the current element.
3. arr: the array currently being iterated over (the entire array).

let nums = [1,2,3,4,5];

nums.map(function(currentValue, idx, arr) {
    return currentValue * currentValue;
}); // [ 1, 4, 9, 16, 25 ]

Skill Drill

1. Create an array of numbers named nums that stores the values 1-10.

2. Use forEach to print out the current index, followed by the number stored there.

3. Use map to add a ! to the end of each of the numbers in the nums array.

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