Some methods of arrays

Iteration of arrays can be done via use of a map function instead of using for loops. Given an array, each element of the array is operated upon. Function creates a new array instead of updating the existing one.

// Multiply each element of oldArray with 3 to create a newArray
var newArray = oldArray.map(function(val) {
    return val * 3;
});

I have to add all numbers in an array. What I have learned since beginning is use a for loop to iterate over the length of an array while adding them to an accumulator variable. Similar operation can be performed using the reduce function. currentElement as the name denotes is the item of the array currently iterated.

// Sum using reduce function with default value of 'sum' is set to '0'
var sum = array.reduce(function(sum, currentElement) {
    return sum + currentElement;
}, 0);

The filter method is used to query arrays and filter out elements depending on the condition specified. This is equivalent to iterating through all elements using for and then using a conditional statement, depending on the results of which the elements are stored in a vector or array or some other data structure. A new array is created when this is used.

// I need only elements that are less than 5 i given array.
var oldArray = [1,2,3,4,5,6,7,8];
var newArray = oldArray.filter(function(val){
    return val < 5;
});
// newArray = [1,2,3,4]

The sort method sorts the elements of the array and the array being operated upon is altered. No copy of the existing array is made. Arrays of strings can also be sorted.

// Sort elements of array in ascending order, 'a' and 'b' are elements being compared at a time
array.sort(function(a, b){
    return a-b;
    // If sorting in descending order, return b-a
});

It is to be noted that a negative value returned implies element ‘a’ is smaller than ‘b’ and hence should be assigned a smaller index. A value greater than 0 implies the larger index value should be assigned to ‘a’ and when equal to 0, means both ‘a’ and ‘b’ are equal.

reverse reverses the elements of the array.

var array = [1,2,3];
var reverseArray = array.reverse()

concat is used to merge the contents of two arrays into one. It takes an array as an argument and returns a new array with the elements of this array concatenated onto the end of the array on which this operation is applied.

var array = [1,2,3];
var concatArray = [4,5,6];
var newArray = array.concat(concatArray);
// newArray = [1,2,3,4,5,6];

I have a string and I want to tokenize it into different elements and save that in an array. split does this depending on the delimiter passed in as argument to this function.

var stringToSplit = "I am learning how to split";
var array = stringToSplit.split(' '); // space is used as delimiter here.

On splitting, array has 6 elements for each word in the string.

Next function is join which joins the elements of the array to form a string.

var hobbies = ["reading", "writing", "drawing"];
var hobbiesStr = hobbies.join(" and ");
console.log(hobbiesStr); // reading and writing and drawing

These are few of the methods available for arrays which ease up performing the corresponding functions without having to iterate and compare or append manually. So JS makes these things easier with the pre-defined functions.

Objects in JS

Objects in JS can be created by:

  1. Use of variables
  2. Use of functions
// table variable                      |              // use of function
var table = {                          |              var table = function() {
    "legs":4,                          |                  this.legs = 4;
    "color":"brown",                   |                  this.color = "brown";
    "material":"wood"                  |                  this.material = "wood";
};                                     |              };

An instance of these objects can be created as:

var myTable = new table();

Arguments can be passed as needed in the constructor with corresponding changes in definition of the constructor. All the variables defined as property of this are public. In order to make a variable private, var keyword is used. Private variables are created to control changes that happen to their values from outside. Functions to modify these values are then defined within the constructor. This is illustrated in Bike object below.

var car = function() {
    var speed = 20;            // private variable

    // public methods below
    this.accelerate = function(paceup) {
        speed += change;
    };

    this.decelerate = function() {
        speed -= 5;
    };

    this.getSpeed = function() {
        return speed;
    };
};

Hour 2 : JS

Continuing learned using boolean values, conditional statements, switch case and use of functions. Everything goes on like we might have learned in some other language. Encountered only a new operator category called the strict equality operator and strict inequality operator.

Strict equality operator uses === instead of the normal == for the equality operator. So is the case for strict inequality operator which uses !== and not !=. These strict comparison operators compare the types of values being compared unlike the normal comparison operator that coerces the data type and checks only the values. No more differences than standard practices of these operators and conditionals!