JavaScript Variable Manipulation Functions
As shown in the following table, you can use these JavaScript statements in your own code to create and modify variables in your JavaScript functions.
| Element | Description |
|---|---|
| var myVar = 0; | Creates a variable with given starting value. Type is determined dynamically. |
| stringVar = prompt(“message”) | Sends message to user in a dialog box, retrieves text input from user and stores it in stringVar. |
| stringVar.length | Returns the length (in characters) of stringVar. |
| stringVar.toUpperCase(), stringVar.toLowerCase() | Converts stringVar to upper- or lowercase. |
| stringVar.substring() | Returns a specified subset of stringVar. |
| stringVar.indexOf() | Returns location of a substring in stringVar (or -1). |
| parseInt() | Converts string to int. |
| parseFloat() | Converts string to float. |
| toString() | Converts any variable to string. |
| eval() | Evaluates string as JavaScript code. |
| Math.ceil() | Converts any number to integer by rounding up. |
| Math.floor() | Converts any number to integer by rounding down. |
| Math.round() | Converts any number to integer by standard rounding algorithm. |
| Math.random() | Returns random float between 0 and 1. |
Basic I/O Commands in JavaScript
JavaScript programmers commonly use the commands shown in the following table for controlling dialog-based input and output in programs to be used on the Web.
| Element | Description |
|---|---|
| alert(“message”); | Creates a popup dialog containing “message.” |
| stringVar = prompt(“message”) | Send message to user in a dialog box, retrieve text input from user and store it in stringVar. |
JavaScript Conditions and Branching Code Structures
Look to the following table for JavaScript control structures you can use in your program code to add branching and looping behavior to your JavaScript programs.
| Element | Description |
|---|---|
| if (condition){ // content } else { // more content } // end if |
Executes content only if condition is true.
Optional else clause occurs if condition |
| switch (expression)
case: value; //code break; default: //code |
Compares expression against one or more values. If expression is equal to value, runs corresponding code. Default clause catches any uncaught values. |
| for(i = 0; i < count; i++)
//code } // end for |
Repeats code i times. |
| While (condition){
//code } // end while |
Repeats code as long as condition is true. |
| Function fnName(paramaters) {
//code } // end function |
Defines a function named fnName and sends it parameters. All code inside the function will execute when the function is called. |
Add JavaScript Comparison Operators to Condition Statements
JavaScript uses comparison operators inside conditions to make numeric or alphabetical comparisons of variables to other variables or values. Using these operators, you can determine whether a variable is greater than, less than, or equal to another variable or value. You can also use combinations of these comparison operators.
| Name | Operator | Example | Notes |
|---|---|---|---|
| Equality | == | (x==3) | Works with all variable types, including strings. |
| Not equal | != | (x != 3) | True if values are not equal. |
| Less than | < | (x < 3) | Numeric or alphabetical comparison. |
| Greater than | > | (x > 3) | Numeric or alphabetical comparison. |
| Less than or equal to | <= | (x <= 3) | Numeric or alphabetical comparison. |
| Greater than or equal to | >= | (x >= 3) | Numeric or alphabetical comparison. |
Create JavaScript Structures and Objects
JavaScript allows you to put together code lines to create functions and variables to create arrays. You can put functions and variables together to create objects.
| Element | Description |
| function fnName(parameters) { //code } // end function |
Defines a function named fnName and sends it parameters. All code inside function will execute when the function is called. |
| var myArray = new Array(“a”, “b”, “c”); |
Creates an array. Elements can be any type (even mixed types). |
| Var myJSON = {
“name”: “title”: } |
Creates a JSON object. Each element has a name/value pair, and can contain anything, including an array (with square braces) another JSON object, or a function. |
| Var person = new Object();
Person.name = |
Creates an object. You can add ordinary variables (which become properties) or functions (which become methods). |
Change Your Web Page with JavaScript Document Object Model Methods
The Document Object Model methods shown in the following table offer you a great way to access and modify your Web pages through your JavaScript code.
| Element | Description |
| myElement = document.getElementById(“name”); |
Gets an element from the page with the specified ID and copies a reference to that element to the variable myElement. |
| myElement.innerHTML = “value” |
Changes the value of the element to “value”. |
| document.onkeydown = keyListener | When a key is pressed, a function called keyListener is automatically activated. |
| document.onmousemove = mouseListener |
When the mouse is moved, a function called mouseListener is automatically activated. |
| setInterval(function, ms); | Runs function each ms milliseconds. |
| myArray = document.getElementByName(“name”) |
Returns an array of objects with the current name (frequently used with radio buttons). |
