WTF JavaScript tutorial helps newcomers get started with JavaScript quickly.

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In this lecture, we will introduce the most common complex type in Javascript: array (Array). It can store multiple data together in an orderly manner.

Array types support storing multiple elements under a single variable name. The simplest way to create an array is to use a comma-separated list inside a pair of square brackets [], for example:

// Empty array with no elements
const empty = [];
//Array to store three strings
const courses = ["Solidity", "Etherjs", "JavaScript"];
// Store arrays of different data types
const mix = [1,'WTF',true];

The elements in the array can be of basic type or another array. You can store more complex nested data through arrays. Below, the complex array contains the courses and mix arrays:

// complex is an array containing two elements, each element is another array
const complex = [courses, mix];
const nested = [[1, 2], [1, 1, 1]];

We can use indexing (brackets []) on variables to read array elements. Note that array indexing starts at 0:

const arr = [1,2,3,["Solidity",true]]
//Read the 0th element
console.log(arr[0]) // 1
//Read the 0th element of the nested array
console.log(arr[3][0]) // "Solidity"

You can modify an array by assigning a value to an element using an index:

//Modify the first element of the array
arr[1] = 6
console.log(arr) //[1, 6, 3, Array(2)]

Array variables also have some properties that can be read. For example, the length property returns the length of the array:

//Output array length
console.log(arr.length) // 4

An array stores multiple data, and we can use a loop to traverse all elements inside the array. In the following example, we use a for loop to calculate the average of the numArr array.

const numArr = [5, 8, 9, 11, 55];
let average = 0;
for(let i = 0; i < numArr.length; i++){
   average += numArr[i] / numArr.length
}
console.log(`average is: ${average}`) // 17.6

There are many methods built into the array, among which push and pop are used to add and delete elements.

In the array, we can use the push method to push (add) an element at the end of the array. This method will increase the length of the array by one.

const nums = [1,2,3]
nums.push(4)
console.log(nums) // [1, 2, 3, 4]

We can pop an element from the end of an array using the pop method. This method decrements the length of the array by one and returns the popped element.

// The pop method will return the popped element
const last = nums.pop()
console.log(last) // 4 pop up
console.log(nums) // [1, 2, 3]

For more array operation methods, please refer to [MDN Tutorial](https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array#%E5%AE%9E%E4%BE% 8B%E6%96%B9%E6%B3%95), you can focus on the slice, splice, indexOf methods.

Complete the sumOdd function below to complete the logic: Calculate the sum of all odd numbers in the input array arr.

function sumOdd(arr){

}

console.log(sumOdd([1, 2, 3, 4, 5])) // should return 9
console.log(sumOdd([2, 4, 6, 8, 10])) // should return 30
console.log(sumOdd([1, 3, 5, 7, 9])) // should return 25

In this lecture, we introduced arrays in Javascript, including array definition, reading, writing, and traversing. Arrays are data structures often used in daily development. We will continue to see them in subsequent tutorials, so be sure to master them well.