Here’s how a rookie reads a book (Hold on, don’t laugh)

This article is used for the rookie level like me, big guy see in the code is not easy, give a praise (welcome to point out the wrong and guidance), some of their own writing, some of the trouble to copy, this article is mainly used for later content review

Updated continuously ~

Js based

• 1. Variable scope — global and local variables

  var str1 = 'global';
  str2 = 'global';
  function myFunction() {
      var str1 = 'local';
      return str1;
  }
  function myOtherFunction() {
      str2 = 'local';
      returnstr2; } console.log(str1); //{1} console.log(myFunction()); //{2} console.log(str2); //{3} console.log(myOtherFunction()); //{4} console.log(str2); / / {5}Copy the code

  {1}– output global because it is a global variable

  {2}– output local, str1 is declared in myFunction, so it only works in the scope of the function itself

  {3}– output global,str2 is assigned to the second line

  {4}– print local, because without var it is equivalent to str2 making a global assignment in its own function scope

  >>> The difference between variables and attributes: the former can not be deleted, the latter can be deleted! For example, delete str1 cannot be deleted, but delete str2 can be deletedCopy the code

  {5}– str2 is local because {4} has been reassigned

  Note: Minimize global variables in your code, or use lets

  ---

• 2. Operators — ternary operators

  Ternary operator

         let a = 1
         let b = 2
         let c = 3
         letd = 4 //1. p = a == 1 ? A: b //a = 1, p=a, otherwise p=b //2. Case, but it's hard to understand p = a == B, right? 1 : (a == c ? a : (a == d ? A: d)) // whether a is equal to b, if so,p= 1. If not, whether A is equal to C, if so,p=a. If not, is a equal to d, if p is equal to a, if not p is equal to dCopy the code

  ---

• True or false — here directly posted for reference

  Numeric types	Converts to a Boolean value
  undefined	false
  null	false
  Boolean value	True is true, false is false
  digital	+0, -0, and NaN are false, and all others are true
  string	False if the string is empty (length 0), true for everything else
  object	true

An array of

1. Fibonacci numbers

Given that the first number is 1, the second number is 2, and the third number is the sum of the first two numbers, the list of the first 20 numbers is given arr[0] = 1, arr[1] = 2; Arr [I] = arr[i-1] + arr[i-2] then it is easy:Copy the code

        letArr = [1, 2]for (let i=2; i<20; i++) {
            arr[i] = arr[i-1] + arr[i-2]
        }
Copy the code

As a result,

2. Component insert value

Inserting a value at the beginning of an array is easy if you use unshift, just arr. Unshift (number) now implements this in the original way, by moving all the values back one bit and assigning arr[0]Copy the code

     let arr = [1, 2, 3, 4, 5]
     let newArr = [10]
     for (leti = arr2.length; i>0 ; i--) { newarr[i] = arr2[i-1] }Copy the code

Results:

3. Multidimensional arrays

Borrow a picture first:

The traditional way to merge a two-dimensional array into a one-dimensional array is to get all the array values and then assign them to a new array. For (var I =0; i<arr.length; i++) { for (var j=0; j<arr[i].length; J ++) {newarr.push (arr[I][j])}} // More multidimensional arrays can be used in the same wayCopy the code

Var newArr=[] arr.foreach (item => {newArr=[...newArr,...item]) var newArr=[] arr.foreach (item => {newArr=[...newArr,...item])Copy the code

for

Explore other array methods on your own! Related array methods: