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COSC 330
 

APPENDIX B TO LM 330-VII
JAVASCRIPT KEYWORDS AND OPERATORS

         This appendix is an adjunct to Appendix A that gives details of keywords that are so intuitive, especially to students with experience developing software in C++ or Java.  They are needed so that the appendices to LM 330 can be complete, as far as Core Javascript is concerned.  They are not included in Appendix A because they are intuitive and add unnecessary complexity; instead, Appendix A is simplified by incorporating references to the details presented here.

1. CORE JAVASCRIPT KEYWORDS:
 

2. CORE JAVASCRIPT OPERATORS:

  1. Operators take one or more variables or values (operands) and return a new value; e.g. the '+' operator can add two numbers to produce a third. You use operators in expressions to relate values, whether to perform arithmetic or compare quantities. Operators are divided into several classes depending on the relation they perform:
    1. Arithmetic operators take numerical values as operands and return a single result. JavaScript has the standard binary arithmetic operators (+, -, *, / and %) for addition, subtraction, multiplication, division and modulus (remander of division).
    2. Unary negation operator which returns the negative of the operand, e.g. -5 is the negative of 5.
    3. Increment and decrement operators.

      4.  
      ANSI STANDARD C INCREMENT AND DECREMENT OPERATORS
      ++ causes a unary increment, i.e. the operand's value is increased by 1.  This operator has only one operand.  The value returned depends on its orientation relative to the operand, e.g. ++x will return the value of x after the increment and x++ will return the value of x before the increment.  (This arcane notation (to me) serves no purpose other than to confuse the novice reader (Originally, in C, is supposedly was more processor efficient.) so I prefer to use the explicit syntax, e.g. replace x++ with x = x +1.)
      -- causes a unary increment, i.e the reverse of ++. (See the preceding comment.)
SAQ: (a) How would one explicitly write out x++?  (b) How is this useful?
    1. A comparison operator compares its two operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical (alphabetic) ordering.
     
    ANSI STANDARD C COMPARISON OPERATORS 
    == "Equal to" returns true if operands are equal.
    != "Not equal to" returns true if operands are not equal.
    > "Greater than" returns true if left operand is greater than right operand.
    >= "Greater than or equal to" returns true if left operand is greater than or equal to right operand.
    < "Less than" returns true if left operand is less than right operand.
    <= "Less than or equal to" returns true if left operand is less than or equal to right operand.
SAQ: Why does JavaScript (like, C, C++, and Java) use "= =" instead of the more sensible "=" for "equals to"?
    1. Boolean operators are typically used to combine multiple comparisons into a conditional expression. For example, you might want to test whether (total>100) AND (stateTax=true). A boolean operator takes two operands, each of which is a true or false value, and returns a true or false result.
     
    && "And" returns true if both operands are true.
    || "Or" returns true if either operand is true.
    ! "Not" returns true if the negation of the operand is true (e.g. the operand is false).
    1. Strings can be compared using the comparison operators. Additionally, you can concatenate strings using the + operator.
     

    + (string concatenation) combines two strings "Hello " + "World" yields "Hello World"
      JavaScript supports regular expressions, which are defined patterns used to match character combinations appearing in string values. Regular expressions are very powerful, potentially allowing you to search for any conceivable character pattern. However, they can also be quite complex to construct. Because regular expressions are widely supported in all high-level development environments, it is advised that you consider learning about regular expressions as a subject unto itself.  Two detailed explanations of regular expressions can be found at
    1. The assignment operator (=) lets you assign a value to a variable. You can assign any value to a variable, including another variable (whose value will be assigned). Several shorthand assignment operators allow you to perform an operation and assign its result to a variable in one step.
     
    = Assigns the value of the righthand operand to the variable on the left.
    Example: total=100;
    Example: total=(price+tax+shipping)
    +=
    (also -=, *=, /=)    		 Adds the value of the righthand operand to the lefthand variable and stores the result in the lefthand variable.
    Example: total+=shipping (adds value of shipping to total and assigned result to total)
    &=
    (also |=)    		 Assigns result of (lefthand operand && righthand operand) to lefthand operand.
SAQ: This use of "=" causes much of the strange arithmetic notation in C, C++, Java, and JavaScript.  Explain.
    1. Several JavaScript operators, rarely used, fall into no particular category. These operators are summarized below.
     
    Conditional operator
    (condition) ? trueVal : falseVal 
    		 Assigns a specified value to a variable if a condition is true, otherwise assigns an alternate value if condition is false.
    Example:
    preferredPet = (cats > dogs) ? "felines" : "canines"
    If (cats>dogs), preferredPet will be assigned the string value "felines," otherwise it will be assigned "canines". 
    NOTE: The IF-ELSE construct (See section ???.) can be used instead of the conditional operator.  Usually this construct is preferred because it produces more readable code.
    typeof operand Returns the data type of operand. 
    Example -- test a variable to determine if it contains a number:
    if (typeof total=="number") ...
  1. OPERATOR PRECEDENCE....