# Looping over arrays

There’re few methods for looping over arrays in Javascript. We’ll start with the classical ones and move towards additions made to the standard.

## while

``````let index = 0;
const array = [1,2,3,4,5,6];

while (index < array.length) {
console.log(array[index]);
index++;
}
``````

## for (classical)

``````const array = [1,2,3,4,5,6];
for (let index = 0; index < array.length; index++) {
console.log(array[index]);
}
``````

## forEach

``````const array = [1,2,3,4,5,6];

array.forEach(function(current_value, index, array) {
console.log(`At index \${index} in array \${array} the value is \${current_value}`);
});
// => undefined
``````

## map

The last construct was useful, however, it doesn’t return a new array which might be undesirable for your specific case. `map` solves this by applying a function over every element and then returning the new array.

``````const array = [1,2,3,4,5,6];
const square = x => Math.pow(x, 2);
const squares = array.map(square);
console.log(`Original array: \${array}`);
console.log(`Squared array: \${squares}`);
``````

The full signature for `map` is `.map(current_value, index, array)`.

## reduce

From MDN:

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

``````const array = [1,2,3,4,5,6];
const sum = (x, y) => x + y;
const array_sum = array.reduce(sum, 0);
console.log(`The sum of array: \${array} is \${array_sum}`);
``````

## filter

Filters elements on an array based on a boolean function.

``````const array = [1,2,3,4,5,6];
const even = x => x % 2 === 0;
const even_array = array.filter(even);
console.log(`Even numbers in array \${array}: \${even_array}`);
``````

## every

Got an array and want to test if a given condition is met in every element?

``````const array = [1,2,3,4,5,6];
const under_seven = x => x < 7;

if (array.every(under_seven)) {
console.log('Every element in the array is less than 7');
} else {
console.log('At least one element in the array was bigger than 7');
}
``````

## some

Test if at least one element matches our boolean function.

``````const array = [1,2,3,9,5,6,4];
const over_seven = x => x > 7;

if (array.some(over_seven)) {
console.log('At least one element bigger than 7 was found');
} else {
console.log('No element bigger than 7 was found');
}
``````