Chapter 18

c h a p t e r

18

Fundamental Concepts Expressed in JavaScript

lawrence snyder

Get with the Program

Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Sunday, November 27, 2011

Overview: Programming Concepts • Programming: Act of formulating an algorithm or program • Basic concepts have been developed over last 50 years to simplify common programming tasks • Concepts will be expressed in JavaScript – Fully general programming language – Designed for making active web pages/apps



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Programming Concepts Not enough for fancy web pages, but these provide a basic set of first capabilities • Names, values, variables • Declarations • Data types, numbers, string literals and Booleans • Assignment • Expressions • Conditionals



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Names, Values, And Variables • A Name is a symbol that represents something • Names Have Changing Values – The name U.S. President has current value of Barack Obama, previous values of Bill Clinton, George Washington

• Names in a Program Are Called Variables – The term “variable” reminds us that the value associated with the symbol can vary during program execution – Values associated with a variable change in programs using the assignment statement ( = ) Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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Names and Changing Values



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Identifiers and Their Rules • An Identifier is the character sequence that is a variable's name • Must be formed following specific rules – Must begin with a letter or underscore ( _ ) followed by any sequence of letters, digits, or underscore characters – Cannot contain spaces – Case sensitive (Capitalization matters!)



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Identifiers and Their Rules Valid

firstOne first1 First_1 First_One fIRSToNE _special very_long_name_ok

Invalid 1stOne first-1 first$1 first One First1! 5 happy:)

For each invalid identifier, what rule is broken? Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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A Variable Declaration Statement • Declaration: State what variables will be used – Command uses the keyword var – For example, a program to calculate area of a circle given a radius, might use variables area and radius: • var radius, area;

• The variable declaration is a type of statement • Can have many var statements in a program



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The Statement Terminator • A program is a list of statements – Several statements may be run together on a line

• Each statement is terminated by the statement terminator symbol – In JavaScript, this is the semicolon ( ; )

• Forgetting the semi-colon is a very common error, so be warned (and wary)



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Rules for Declaring Variables • Every variable used in a program must be declared (before it is used) – In JavaScript declaration can appear anywhere in the program – Programmers prefer to place them first (“up top”)

• Undefined values – A variable may be declared, but may not yet have any value associate with it (its initial value) var speed; speed = 42;

// undefined initial value // now it has a value



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Initializing a Declaration • We can set an initial value as part of a declaration: – var speed = 42;

• Related variables may be grouped in one declaration/initialization; unrelated variables are usually placed in separate statements var numA=27, numB, numC;

var numA = 27; var numB; var numC;

• Since this is what programmers commonly do, but it is not required, it is called a programming convention Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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Three Basic Date Types in JavaScript • In JavaScript, values are grouped into related categories called data types, or just types – numbers (or numeric, for arithmetic) – strings (used for text) – Booleans (used for logic)



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Rules for Writing Numbers • There are no "units" or commas • Can have about 10 significant digits and can range from 10-324 to 10308 • Values with no decimal point are integers 10

1567238

-487

-776734551

0

• Values with a decimal point are real, or floating point 10.0 3.1415926 -478934.4 0.000101101111



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Strings • Strings are sequences of keyboard characters • Strings are always surrounded by single ( ' ' ) or double quotes ( " " ) • Strings can initialize a declaration – var hairColor = “black”, sign=‘Leo’; – var greeting = “Hello, how are you today?”

• Note that the string value “123” is not the same as the number value 123



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Rules for Writing Strings in • Must be surrounded by single or double quotes • Allow most characters except return (Enter), backspace, tab, \ • Double quoted strings can contain single quoted strings and vice versa ( “My dog ‘Spot’ is a yellow Lab” ) • The apostrophe ( ' ) is the same as the single quote



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Rules for Writing Strings in JavaScript • Any number of characters allowed in a string • Minimum number of characters is zero ( "" ), which is the empty string; it contains no characters at all • Quotes do not count in figuring a string’s length – Empty string has length 0, not 2

• Note that the empty string is not the same as the string “ ”, which contains one blank Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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Literals • Literal is the term for a string or number value that is typed out in the program text • String Literals stored in the computer – Quotes are removed (they are only used to delimit the string literal) – Any character can be stored in memory • Even a character that cannot be typed can be stored, using escape mechanism – in JavaScript, the backslash (\) • var twoTabs = “\t\t”, singQuote=“\’”



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Boolean Values • Two logical values: true and false • They are values, not identifiers or strings – true is a Boolean value like 5 is a number value

• Used implicitly throughout the programming process; only occasionally for initializing variables – Mostly used to compare data or make decisions – var done=false, simpleMode=true, speed=42;



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The Assignment Statement • Used to change a variable's value ;

• Assignment Symbol: – In JavaScript, the equal sign ( = )



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Interpreting an Assignment • Value “flows” from the right side to the left side • Read the assignment symbol as "is assigned" or "becomes" or "gets“ speed = 42

we say “speed gets 42”

• The expression (right side) is computed or evaluated first – If there are any variables in it, their current value is used • Then this computed value becomes the value of the variable on the left side



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Three Key Points about Assignment • All three of the components must be given – if anything is missing, the statement is meaningless • Flow of value to name is always right to left • Values of any variables used in the expression are always their values before the start of the execution of the assignment



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A Fourth Key Point about • Programming is not Algebra • Consider this statement x=x+1 – In Algebra, this is impossible… no number is the same as one greater than itself – In programming, we say “x gets x+1” meaning take the value in x, add 1, then store the new value back in x

• Some programming languages do not use ‘=‘ for the assignment symbol to avoid this confusion Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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An Expression and its Syntax • An expression is a math-like formula – Describe the means of performing the actual computation – Built out of values and operators • standard arithmetic operators are symbols of basic arithmetic • relational operators compare values • logical operators and string operators too



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Arithmetic Operators • Add, subtract, multiply, divide ( +, -, *, / ) • Multiplication must be given explicitly with the asterisk ( * ) multiply operator – 2ab works in math, but in JavaScript we write 2*a*b

• Multiply and divide are performed before add and subtract – Unless grouped by parentheses – This is called operator precedence



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Arithmetic Operators • Modulus or mod ( % ) divides two integers and returns the remainder • JavaScript does not have an operator for exponents • Binary operators operate on two operands – like + and * and %

• Unary operators operate on one operand – like - for negation



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Relational Operators • Make comparisons between numeric values • Outcome is a Boolean value, true or false

< less than = greater than or equal to > greater than

• 5 < 10 evaluates to true • 8.54 == 14.7 evaluates to false • speed > 42 have to see what value speed has



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Summary • Names and values are distinct concepts; names of variables must be declared; variables can be initialized when declared; variable values are changed with assignment • An assignment statement has a variable name on the left, an expression on the right, and the assignment symbol in the middle • Assignment works by evaluating the expression on the right (using values currently in the variables) and then putting the new value into the variable on the left Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley


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Summary • JavaScript has 3 data types: number, string, and Boolean • We can build expressions to compute values of these types • Arithmetic operators, and relational operators work on number values; logical operators work on Boolean values • Operator overloading means that sometimes one symbol is used to represent two different operations (on two types)



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