(Music) Hello. Welcome to JavaScript Variables and Control Statements. After watching this video, you will be able to: Explain how variables are declared and used in JavaScript, and describe JavaScript control structures, Variables are declared with the var keyword followed by the variable name, as in the example, var age, Variables can be declared and initialized in one step, as in the example, var age = 54, You can assign a value to the variable at a later time, or reassign the value of a variable, Since JavaScript is a loosely typed language, you do not need to declare the data type of a variable. The variable assumes the data type from the value of the field during assignment and the type of a variable can change during program execution. Beware of not initializing variables though – the value of ‘var age’ – that has not been assigned a value, is not zero, or an empty String, or any other useful default value; it is ‘undefined’. Remember that the only keyword is ‘var’, and JavaScript has no way to know whether this variable is a numeric or a string, or anything else, until you provide a value. It therefore cannot decide by default whether to assign a zero or an empty string. Variable names, or identifiers, have these rules: The name must start with a letter, underscore (_), or a dollar sign ($), Subsequent characters can also be digits [0-9] Identifiers are case-sensitive. Variables also have a scope: Variables declared within a function have a scope local to that function, Variables declared outside of a function have a global scope. Variables with a global scope can be used elsewhere in the JavaScript program, Variables declared without the var keyword have a global scope. Variables that are not initialized have a value of undefined. You learn about functions shortly. Conditional statements are the set of commands that are used to perform different actions for different conditions: In JavaScript, the IF statement is the way the program logic decides which path to take based on the current values of variables or object properties, The JavaScript syntax for a decision begins with the keyword IF, followed by the condition to test; then the statements that run if the condition yields a true result. The true processing follows immediately after the test condition and is delimited by braces, unless it is a single statement. If the condition resolves to false, the statements that follow the else keyword are executed. Indentation of the statements of a compound control statement is not required in JavaScript However, a programmer can find that indentation helps in deciphering the control statement. Many JavaScript aware text editors automatically indent the control structure to make it more readable. Unlike Java, there is no block statement scope in JavaScript. Having no block statement scope means that variables declared inside one IF condition can be used outside the scope of that condition. JavaScript supports the switch statement as an alternative to the IF then ELSE control statements. The condition that is being tested is placed in parentheses that follow the switch keyword. The expression parameter of the switch statement can evaluate to any number or string value. The labels in the case statement are enclosed in quotation marks when the labels represent string values of the expression. The program looks for a case clause with a label that matches the value of expression and then transfers control to that clause, running the associated statements. If no matching label is found, the program looks for the optional default clause, and if found, transfers control to that clause, issuing the associated statements. The break keyword is used to prevent the code from automatically falling into the next case clause. The FOR loop repeats a series of statements for any number of times. The FOR loop takes three parameter arguments, namely the initial value, the condition that is being tested, and the increment expression. When the FOR loop runs, the following occurs: First, the initial expression is set, second, the conditional expression is evaluated. If this condition evaluates to true, the loop statements runs and the increment expression is updated. If the condition evaluates to false, the FOR loop terminates. If the loop does not terminate, control returns to the second step and the conditional expression is evaluated again. The WHILE loop is another common loop in JavaScript, The loop repeats while the condition remains true, The WHILE loop assumes that the condition reaches a conclusion and then exits the loop. Make sure that the condition evaluates to false at some point; otherwise the loop never terminates. In this video, you learned: Variables are declared using the keyword var followed by the variable name. You can initialize variables at the time of declaration or assign a value later. You do not need to declare the data type of a variable. Variables take their data type from the value assigned and can change type during program execution. Variables also have a scope . Variables declared within a function have a scope local to that function. Variables declared outside of a function have a global scope. Variables declared without the var keyword have a global scope. When executing, the flow of the program is directed by control statements, including: Conditional statements like if...then...else, Switch statements, repeat statements like for loops and while loops.
