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Statements

JavaScript statements consist of keywords used with the appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semi-colon.

Syntax conventions: All keywords in syntax statements are in bold. Words in italics represent user-defined names or statements. Any portions enclosed in square brackets, [ ], are optional. {statements} indicates a block of statements, which can consist of a single statement or multiple statements delimited by a curly braces {}.

The following statements are available in JavaScript:

  • break
  • comment
  • continue
  • for
  • for...in
  • function
  • if...else
  • new
  • return
  • this
  • var
  • while
  • with

    • NOTE: new and this are not really statements, but are included in this section for convenience.

    break

    A statement that terminates the current while or for loop and transfers program control to the statement following the terminated loop.

    Syntax

    break

    Examples

    The following function has a break statement that terminates the while loop when i is 3, and then returns the value 3 * x.

    function func(x) { var i = 0 while (i &lt 6) { if (i == 3) break i++ } return i*x }

    comment

    Notations by the author to explain what a script does. Comments are ignored by the interpreter. JavaScript supports Java-style comments:

    • Comments on a single line are preceded by a double-slash (//).
    • Comments that span multiple lines are preceded by a /* and followed by a */.

    Syntax

    1. // comment text
2. /* multiple line comment text */

    Examples

    // This is a single-line comment. /* This is a multiple-line comment. It can be of any length, and you can put whatever you want here. */

    continue

    A statement that terminates execution of the block of statements in a while or for loop, and continues execution of the loop with the next iteration. In contrast to the break statement, continue does not terminate the execution of the loop entirely: instead,

    • In a while loop, it jumps back to the condition.
    • In a for loop, it jumps to the update expression.

    Syntax

    continue

    Examples

    The following example shows a while loop that has a continue statement that executes when the value of i is 3. Thus, n takes on the values 1, 3, 7, and 12.

    i = 0 n = 0 while (i &lt 5) { i++ if (i == 3) continue n += i }

    for

    A statement that creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a block of statements executed in the loop. The parts of the for statement are:

    • The initial expression, generally used to initialize a counter variable. This statement may optionally declare new variables with the var keyword. This expression is optional.
    • The condition that is evaluated on each pass through the loop. If this condition is true, the statements in the succeeding block are performed. This conditional test is optional. If omitted, then the condition always evaluates to true.
    • An update expression generally used to update or increment the counter variable. This expression is optional.
    • A block of statements that are executed as long as the condition is true. This can be a single statement or multiple statements. Although not required, it is good practice to indent these statements from the beginning of the for statement.

    Syntax

    for ([initial expression]; [condition]; [update expression]) {
   statements
}
initial expression = statement | variable declaration

    Examples

    This for statement starts by declaring the variable i and initializing it to zero. It checks that i is less than nine, and performs the two succeeding statements, and increments i by one after each pass through the loop.

    for (var i = 0; i < 9; i++) { n += i myfunc(n) }

    for...in

    A statement that iterates a variable var over all the properties of object obj. For each distinct property, it executes the statements in statements.

    Syntax

    for (var in obj) {
   statements }

    Examples

    The following function takes as its argument an object and the object's name. It then iterates over all the object's properties and returns a string that lists the property names and their values.

    function dump_props(obj, obj_name) { var result = "" for (var i in obj) { result += obj_name + "." + i + " = " + obj[i] + "<BR>" } result += "<HR>" return result }

    function

    A statement that declares a JavaScript function name with the specified parameters param. Acceptable parameters include strings, numbers, and objects.

    To return a value, the function must have a return statement that specifies the value to return. You cannot nest a function statement in another statement or in itself.

    All parameters are passed to functions, by value. In other words, the value is passed to the function, but if the function changes the value of the parameter, this change is not reflected globally or in the calling function.

    Syntax

    function name([param] [, param] [..., param]) {
   statements }

    Examples

    //This function returns the total dollar amount of sales, when //given the number of units sold of products a, b, and c. function calc_sales(units_a, units_b, units_c) { return units_a*79 + units_b*129 + units_c*699 }

    if...else

    A conditional statement that executes the statements in statements if condition is true. In the optional else clause, it executes the statements in else statements if condition is false. These may be any JavaScript statements, including further nested if statements.

    Syntax

    if (condition) {
   statements
} [else {
   else statements
}]

    Examples

    if ( cipher_char == from_char ) { result = result + to_char x++ } else result = result + clear_char

    new

    An operator that lets you create an instance of a user-defined object type.

    Creating an object type requires two steps:

    1. Define the object type by writing a function.
    2. Create an instance of the object with new.

    To define an object type, create a function for the object type that specifies its name, properties, and methods. An object can have a property that is itself another object. See the examples below.

    You can always add a property to a previously defined object. For example, the statement car1.color = "black" adds a property color to car1, and assigns it a value of "black". However, this does not affect any other objects. To add the new property to all objects of the same type, you must add the property to the definition of the car object type.

    Syntax

    objectName = new objectType ( param1 [,param2] ...[,paramN] )
    objectName is the name of the new object instance.
    objectType is the object type. It must be a function that defines an object type.
    param1...paramN are the property values for the object. These properties are parameters defined for the objectType function.

    Examples

    Example 1: object type and object instance.Suppose you want to create an object type for cars. You want this type of object to be called car, and you want it to have properties for make, model, year, and color. To do this, you would write the following function:

    function car(make, model, year) { this.make = make this.model = model this.year = year }

    Now you can create an object called mycar as follows:

    mycar = new car("Eagle", "Talon TSi", 1993)

    This statement creates mycar and assigns it the specified values for its properties. Then the value of mycar.make is the string "Eagle", mycar.year is the integer 1993, and so on.

    You can create any number of car objects by calls to new. For example,

    kenscar = new car("Nissan", "300ZX", 1992)

    Example 2: object property that is itself another object. Suppose you define an object called person as follows:

    function person(name, age, sex) { this.name = name this.age = age this.sex = sex }

    And then instantiate two new person objects as follows:

    rand = new person("Rand McNally", 33, "M") ken = new person("Ken Jones", 39, "M")

    Then you can rewrite the definition of car to include an owner property that takes a person object, as follows:

    function car(make, model, year, owner) { this.make = make; this.model = model; this.year = year; this.owner = owner; }

    To instantiate the new objects, you then use the following:

    car1 = new car("Eagle", "Talon TSi", 1993, rand); car2 = new car("Nissan", "300ZX", 1992, ken)

    Instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the parameters for the owners. To find out the name of the owner of car2, you can access the following property:

    car2.owner.name

    return

    A statement that specifies the value to be returned by a function.

    Syntax

    return expression

    Examples

    The following function returns the square of its argument, x, where x is a number.

    function square( x ) { return x * x }

    this

    A keyword that you can use to refer to the current object. In general, in a method this refers to the calling object.

    Syntax

    this[.propertyName]

    Examples

    Suppose a function called validate validates an object's value property, given the object and the high and low values: function validate(obj, lowval, hival) { if ((obj.value < lowval) || (obj.value > hival)) alert("Invalid Value!") }

    You could call validate in each form element's onChange event handler, using this to pass it the form element, as in the following example:

    <INPUT TYPE = "text" NAME = "age" SIZE = 3 onChange="validate(this, 18, 99)">

    var

    A statement that declares a variable varname, optionally initializing it to value. The variable name varname can be any legal identifier, and value can be any legal expression. The scope of a variable is the current function or, for variables declared outside a function, the current application.

    Using var outside a function is optional; you can declare a variable by simply assigning it a value. However, it is good style to use var, and it is necessary in functions if there is a global variable of the same name.

    Syntax

    var varname [= value] [..., varname [= value] ]

    Examples

    var num_hits = 0, cust_no = 0

    while

    A statement that creates a loop that evaluates the expression condition, and if it is true, executes statements. It then repeats this process, as long as condition is true. When condition evaluates to false, execution continues with the statement following statements.

    Although not required, it is good practice to indent the statements a while loop from the beginning of a for statement.

    Syntax

    while (condition) {
   statements
}

    Examples

    The following while loop iterates as long as n is less than three. Each iteration, it increments n and adds it to x. Therefore, x and n take on the following values:

    • After the first pass: x = 1 and n = 1
    • After the second pass: x = 2 and n = 3
    • After the third pass: x = 3 and n = 6

    After completing the third pass, the condition n < 3 is no longer true, so the loop terminates.

    n = 0 x = 0 while( n &lt 3 ) { n ++; x += n }

    with

    A statement that establishes object as the default object for the statements. Any property references without an object are then assumed to be for object. Note that the parentheses are required around object.

    Syntax

    with (object){
    statements
}

    Examples

    with (Math) { a = PI * r*r x = r * cos(theta) y = r * sin(theta) }