Chapter 2 Data Types and Operators JavaScript, Third Edition.

Chapter 2

Data Types and Operators

JavaScript, Third Edition

JavaScript, Third Edition 2

Objectives

• Work with variables

• Study data types

• Use expressions and operators

• Work with strings

• Study operator precedence


Variables

• Values that a program stores in computer memory

• Similar to a storage locker

• To use in a program:

– Write a statement that creates the variable

– Assign it a name

• Assigning a value to a variable is the same as storing a value in it


Variable names

• Name you assign to a variable is called an identifier

• Rules and conventions when naming a variable:

– Identifiers must begin with an uppercase or lowercase ASCII letter, dollar sign ($), or underscore ( _ )

– You can use numbers in an identifier, but not as the first character

– You cannot include spaces in an identifier

– You cannot use reserved words for identifiers


Variable Names (Cont.)• Reserved words (keywords):

– Special words part of the JavaScript language


Declaring Variables

• In JavaScript, the reserved keyword var is used to create variables

• Creating a variable is called declaring the variable

• Assign a specific value to, or initialize, the variable by using the syntax:

– Var variable_name = value;

• The value assigned to a variable can be a literal string or a numeric value


Declaring Variables (Cont.)

• When assigning a literal string value to a variable:

– Enclose text in quotation marks

• When assigning a numeric value to a variable:

– Do not enclose value in quotation marks

• You can declare multiple variables in the statement using a single var keyword followed by a series of variable names and assigned values separated by commas


Modifying Variables

• You can change the variable’s value at any point in a script:

– Use a statement that includes variable’s name, followed by an equal sign, followed by the value to assign to the variable


Data Types

• Specific category of information that a variable contains

• Helps determine how much memory the computer allocates for data stored in the variable

• Data type also governs the kinds of operations that can be performed on a variable


Data Types (Cont.)• Data types that can be assigned only a single value

are called primitive types • JavaScript supports six primitive data types


Data Types (Cont.)

• The null value

– data type/value that can be assigned to a variable

– Indicates the variable does not contain a usable value

– A variable with a value of “null” has a value assigned to it

• Null is really “no value”

– You assign the “null” value to a variable when you want to ensure that the variable does not contain any data


Data Types (Cont.)

• The JavaScript language also supports reference, or composite, data types

• Reference data types:

– Can contain multiple values or complex types of information

• As opposed to single values stored in primitive data types


Data Types (Cont.)

– Three reference data types supported by the JavaScript language are

• Functions

• Objects

• Arrays


Data Types (Cont.)

• Strongly typed programming languages– Require you to declare the data types of variables

– Also known as static typing• Data types do not change after they have been declared

• Loosely typed programming languages– Do not require you to declare the data types of

variables

– Also known as dynamic typing • Data types can change after they have been declared


Data Types (Cont.)

• JavaScript is a loosely typed programming language

– You are not required, and not allowed, to declare the data type of variables

• JavaScript interpreter

– Automatically determines what type of data is stored in a variable

– Assigns the variable’s data type accordingly


Numeric Data Types

• Are an important part of any programming language

• Are particularly useful for arithmetic calculations

• JavaScript supports two numeric data types

– Integers

– Floating-point numbers


Numeric Data Types (Cont.)• Integer

– Positive or negative number with no decimal places

– Can range from -9007199254740990 (-253) to 9007199254740990 (253)

• Floating-point number– Contains decimal places

• OR

– Written in exponential notation

• Exponential notation, or scientific notation– Shortened format for writing very large numbers or

numbers with many decimal places


Boolean Values

• Logical value of true or false

• Can also be thought of as being yes or no, or on or off

• Most often used for

– Deciding which parts of a program should execute

– Comparing data

• In JavaScript programming, you can only use the words true and false to indicate Boolean values


Boolean Values (Cont.)

• In other programming languages, you can use the integer values of 1 and 0 to indicate Boolean values of true and false

– 1 indicates true and 0 indicates false

– JavaScript converts the values true and false to the integers 1 and 0 when necessary


Arrays

• Contain set of data represented by a single variable name

• Collection of variables contained within a single variable

• Use arrays when you want to store groups or lists of related information in a single, easily managed location


Arrays (Cont.)

• Represented in JavaScript by the Array object:

– Contains a special constructor named Array() which is used for creating an array

• Constructor:

– special type of function used as the basis for creating reference variables


Arrays (Cont.)

• Create new arrays by using the keyword new and the Array() constructor with the following syntax:

– Var arrayName = new Array(numberƒofƒelements);

• Within the parentheses of the Array() construction

– Include an integer to represent the number of elements to be contained in the array

• Each piece of data contained in an array is called an element


Arrays (Cont.)

• The numbering of elements within an array starts with an index number of zero (0)

• Index number:

– An element’s numeric position within the array

• Refer to a specific element by enclosing its index number in brackets at the end of the array name


Arrays (Cont.)• Assign values to individual array elements in the

same fashion as you assign values to a standard variable:

– EXCEPT you include the index for an individual element of the array

• When you create a new array with the Array() constructor:

– Declaring the number of array elements is optional


Expressions and Operators• Expression

– Literal value or variable or a combination of literal values, variables, operators, and other expressions that can be evaluated by the JavaScript interpreter to produce a result

– Use operands and operators to create expressions in JavaScript

• Operands – Variables and literals contained in an expression

• Literal – Value such as a literal string or a number


Expressions and Operators (Cont.)

• Operators – Symbols used in expressions to manipulate operands,

such as the addition operator (+) and multiplication operator (*)

• Binary operator – Requires an operand before and after the operator

• Unary operator – Requires a single operand either before or after the

operator


Expressions and Operators (Cont.)


Arithmetic Operators

• Arithmetic operators

– Used in JavaScript to perform mathematical calculations, such as addition

– Also used to return the modulus of a calculation:

• Remainder left when you divide one number by another number


Arithmetic Binary Operators


Arithmetic Unary Operators


Arithmetic Unary Operators (Cont.)

• The increment (++) and decrement (--) unary operators can be used as prefix or postfix operators

• A prefix operator is placed before a variable

• A postfix operator is placed after a variable


Assignment Operators

• Assignment operators are used for assigning a value to a variable

• JavaScript includes other assignment operators called compound assignment operators:

– Perform mathematical calculations on variables and literal values in an expression, and then assign a new value to the left operand


Assignment Operators (Cont.)


Comparison and Conditional Operators

• Comparison operators:

– Used to compare two operands and determine if one numeric value is greater than another

– A Boolean value of true or false is returned after two operands are compared


Comparison and Conditional Operators (Cont.)


Comparison and Conditional Operators (Cont.)

• Comparison operator is often used with the conditional operator:

– Executes one of two expressions, based on the results of a conditional expression

– The syntax for the conditional operator is conditional expression ? expression1: expression2;


Logical Operators

• Logical operators are used for comparing two Boolean operands for equality


Working With Strings

• A text string:

– Contains zero or more characters surrounded by double or single quotation marks

– Can be used as literal value or can be assigned to a variable

– Literal strings can be also be assigned a zero-length string value:

• Called an empty string


Escape Characters and Sequences

• Escape character

– Tells compiler or interpreter that the character following it has a special purpose

– In JavaScript, the escape character is the backslash ( \ )

– Placing a backslash in front of an apostrophe tells the JavaScript interpreter:

» Apostrophe is to be treated as a regular keyboard character


Escape Characters and Sequences (Cont.)

• the combination of the escape character with other characters is called an escape sequence


String Operators

• Two operators used with strings:

– (+)

– (+=)

• When used with strings, the plus sign is known as the concatenation operator (+):

– Used to combine two strings


Special Operators


Special Operators (Cont.)

• One special operator is the typeof operator

– Useful because data type of variables can change during course of program execution

– Use to determine the data type of a variable

– Syntax is typeof(variablename);


Special Operators (Cont.)


Operator Precedence

• Operator precedence:

– Order in which operations in expression are evaluated

• When performing operations with operators in the same precedence group

– The order of precedence is determined by the operator’s associativity

– That is, the order in which operators of equal precedence execute


Operator Precedence (Cont.)


Chapter Summary

• Variables– Values a program stores in computer memory

• Data type– Specific category of information that a variable

contains • Array

– Contains a set of data represented by a single variable name


Chapter Summary (cont.)

• Constructor

– Special type of function used as basis for creating variables of reference data types

• Operands

– Variables and literals contained in an expression


Chapter Summary (cont.)

• Operators

– Symbols used in expressions to manipulate operands, such as the addition operator (+) and multiplication operator (*)

• Operator precedence

– Order in which operations in an expression are evaluated

Chapter 2 Data Types and Operators JavaScript, Third Edition.

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Transcript of Chapter 2 Data Types and Operators JavaScript, Third Edition.