Javascript

Five Different Kinds of Loops

For

A For loop runs through a block of code until it meets a certain condition. This is a good loop for counting.

for (initialization; condition; update) {
    statements;
}

//Change all links to a certain color
var links = document.querySelectorAll("a");
for (var i= 0; i < links.length; i++) {
    links[i].style.color = "#2477CA";
}

//Change all h2 tags to a certain color
var headings = document.querySelectorAll(
    "main * h2, h3, h4, h5, h6");
    for (var i = 0; i < headings.length; i++) {
    headings[i].style.color = "#244873";
}

For/in

for (var in object) {
    code block to be executed
    }

var car = {
    year:"2010", 
    make:"Ford", 
    model:"Mustang"};

    var text = "";
    var x;
    for (x in car) {
        text += car[x];
    }

For/of

The For/of loops lets you loop through a variety of structures including arrays, strings, maps, and nodelists. An interable is something that you can step through the properties as a collection. If you would like to display the all items from an object or array, change the = to +=.

for (variable of iterable) {
    // code block to be executed
    }

var cars = ['BMW', 'Volvo', 'Mini'];
    var x;
    
    for (x of cars) {
        document.write(x + "
");
    }

While

while (condition) {
    // code block to be executed
    }

var i = 1;
    while (i < 3) {
        window.alert(i);
        i++;
    }

Do/while

Do/while Loop is the only post-test loops. All others are pre-test loops. The do statements will run as long as the conditions are met for while.

var i = 1;
    while (i < 3) {
        window.alert(i);
        i++;
    }

3 Different Kinds of Conditional Statements

1. if statement
2. if...else statement
3. if...else if...else statement

Syntax

if (condition) {
    // code executed if condition is true
    } 
else {
    // code executed if condition is false
}

Example

function testNum(a) {
    if (a > 0) {
        return "positive";
    } 
    else {
        return "NOT positive";
    }
}
    
    console.log(testNum(-5));
    // expected output: "NOT positive"

Syntax

function name(
    parameter1, 
    parameter2, 
    parameter3) {
    // code to be executed
}

Example

// Function called. Return value is x
var x = myFunction(4, 3);   

function myFunction(a, b) {
    // Function returns a times b
    return a * b;             
}

JavaScript Identifiers Naming Rules

• Names can contain letters, digits, underscores, and dollar signs.
• Names must begin with a letter, &, or _
• Names are case sensitive
• Reserved words (like JavaScript keywords) cannot be used as names

var x = 5;
    var costOfMeal = $20.15
    var text = document.getElementById(
        'celsiusInputBox').value;

JavaScript Variable Types

Example

function greetUser() {
    var name = window.prompt(
        "Please enter your name");
    var greeting = "Hello " 
        + name + ". 
        Welcome to my site!";
    window.alert(greeting);
}

Declaring and Creating Arrays

Arrays are used to store multiple values together into one variable and can be strings or numbers. There are several ways to declare an array.

Declare an array:
var arrayName;

Create an array:
arrayName = new Array(size);
colors = new Array(6);

Declare and create array:
var arrayName = new Array(size);
var colors = new Array(6);

Array example:
var colors = ["Blue",
    "Red",
    "Green",
    "Yellow"];

Accessing an Array

All values in an array are given an index number. Arrays start at 0 and count up. To access an array, you write the array name and the index number with square brackets.

Example

var colors[3];