add new tip

_posts/en/javascript/2017-06-15-looping-over-arrays.md

@@ -0,0 +1,122 @@
+---
+layout: post
+
+title: Looping over arrays
+tip-number: 79
+tip-username: loverajoel
+tip-username-profile: https://github.com/loverajoel
+tip-tldr: There's a few methods for looping over arrays in Javascript. We'll start
+with the classical ones and move towards additions made to the standard.
+
+categories:
+    - en
+    - javascript
+---
+
+# Looping over arrays
+
+There's a few methods for looping over arrays in Javascript. We'll start
+with the classical ones and move towards additions made to the standard.
+
+## while
+
+```javascript
+let index = 0;
+const array = [1,2,3,4,5,6];
+
+while (index < array.length) {
+  console.log(array[index]);
+  index++;
+}
+```
+
+## for (classical)
+
+```javascript
+const array = [1,2,3,4,5,6];
+for (let index = 0; index < array.length, index++) {
+  console.log(array[index]);
+}
+```
+
+## forEach
+
+```javascript
+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.
+
+```javascript
+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: ${array}`);
+```
+
+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.
+
+```javascript
+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.
+
+```javascript
+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?
+
+```javascript
+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.
+
+```javascript
+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');
+}