[Javascript] Array & Object

Array

• Use index to access individual element:

  var a1 = [2, 5, 6, 8];
  
  var h1 = a1[0];  //=> h1 = 2
  var h2 = a1[2];  //=> h2 = 6
  

• Use array's length property to check the number of element in an array

  var a1 = [2, 5, 6, 8];
  
  var h3 = a1.length //=> h3 = 4

• Use array's forEach method to do something with each array element

  var a1 = [2, 5, 6, 8];
  
  var printIt = function ( x ) {
    console.log(x);
  };
  
  a1.forEach( printIt );
      // 1. forEach expects the argument,
      //    which is printIt in this example,
      //    being a function accepting an argument.
      // 2. Each array's element is used as argument to call "printIt"
      //    i.e. printIt(2), printIt(5), printIt(6), printIt(8)
  //=> 2
  //=> 5
  //=> 6
  //=> 8
  

• More compact form (save the variable printIt)

  var a1 = [2, 5, 6, 8];
  
  a1.forEach( function ( x ) {
    console.log(x);
  });
  //=> 2
  //=> 5
  //=> 6
  //=> 8
  

 Example: (Array and Function)

Function calculating the average of the elements of an array of numbers

    // Define a function
    var average = function (array) {
        var sum = 0;

        array.forEach( function(x) {
            sum = sum + x;
        });

        return sum/array.length;
    }

    var a = [1, 2, 3, 4];

    // Call the function with an array and save the returned value
    var result = average(a);

    console.log(result);
    //=>2.5

 Object

• as collection of key/value (or property/value) pairs
  Example:

    object = { key1:value1, key2:value2, ... };
    

• use dot notation to access the value of an object's property
  Example:

    object.key1
    

• array of objects
  i.e.

    [ {  }, {  }, ... ]
    

• Example:

    Array:
    var book   = [ "JavaScript", "CSS", "HTML"];
    var year   = [ "2010", "2012", "2015"];
    var price  = [ 350, 240, 138];
  
    Object:
    var book1 = {"bookName":"JavaScript", "pubYear":"2010", "listPrice":350};
    var book2 = {"bookName":"CSS", "pubYear":"2012", "listPrice":240};
    var book1 = {"bookName":"HTML", "pubYear":"2015", "listPrice":138};
  
    Array of Objects:
    var books = [ {"bookName":"JavaScript", "pubYear":"2010", "listPrice":350},
                  {"bookName":"CSS", "pubYear":"2012", "listPrice":240},
                  {"bookName":"HTML", "pubYear":"2015", "listPrice":138}
                 ];
  
     i.e.  var books = [ book1, book2, book3 ];
  
    books[1].bookName
      //--> "CSS"