Array .forEach() / map() Since: ES5（ECMAScript 2009）

// Executes a function for each element in an array.
arr.forEach(function(element, index, arr) {
	// process
});

// Transforms each element and creates a new array.
var newArray = arr.map(function(element, index, arr) {
	return transformedValue;
});

var numbers = [1, 2, 3, 4, 5];

// Print each element using forEach.
numbers.forEach(function(num, index) {
	console.log(index + ": " + num);
});

// Use map to create a new array with each element doubled.
var doubled = numbers.map(function(num) {
	return num * 2;
});
console.log(doubled); // Outputs "2,4,6,8,10".
console.log(numbers); // Outputs "1,2,3,4,5". The original array is unchanged.

// Use map to convert elements to strings.
var labels = numbers.map(function(num) {
	return "No." + num;
});
console.log(labels); // Outputs "No.1,No.2,No.3,No.4,No.5".

Both array.forEach() and array.map() execute a function for each element in an array, but they serve different purposes. array.forEach() is used simply to loop through elements and returns no value. array.map(), on the other hand, transforms each element to build a new array and does not modify the original array.

The callback function receives three arguments: the first is the value of the current element, the second is its index, and the third is the array itself. The second and third arguments are optional and can be omitted if not needed.

Unlike a for loop, array.forEach() provides no way to stop the loop early. The break statement cannot be used, and a return statement only skips the current iteration. If you need to stop when a condition is met, use a for loop or consider using array.some().