Chapter 1 Introduction to JavaScript

1.JS is a high-level、dynamic、untyped interpreted programming
   laguage that is well-suited to object-orented and functional
   programming style.
2.

    // variable is a symbolic name for a value.
    var x;          // Declar a variable named x.
    x = 0;
    x

    x = 1;
    x = 0.01;
    x = "hello world";
    x = 'JavaScript';
    x = true;   // Boolean values.
    x = false;

    x = null;   // Null is a special value that means "no value".
    x = undefined;   // Undefined is like null.
   

3.Two other very importy types that JavaScript programs can
   maipulate are object and arrays.
  

    // JavaScript's most important data type is the object.
    // An object is a collection of name/value pairs, or a string to
    //     value map.
    var book = {      // Objects are enclosed in curly braces.
        topic: "JavaScript", // The property "topic" has value "JavaScript".
        fat: true     // The propery "fat" has value true.
    };
    // Access the properties of an object with . or[]:
    book.topic               // => "JavaScript"
    book["fat"]              // => true: another way to access property values.
    book.author = "Flanagan"; // Create new properties by assignment.
    book.contents = {};       // {} is an empty object with no properties.

    // JavaScript also supports arrays (numerically indexed lists) of values:
    var primes = [2. 3, 5, 7];   // An array of 4 values, delimited with [ and ].
    primes[0]              // => 2: the first element (index 0) of the array.
    primes.length          // => 4: how many elements in the array.
    primes[primes.lenght-1]  // => 7: the lastelement of the array.
    primes[4] = 9;           // Add a new element by assignment.
    primes[4] = 11;          // Or alter an existing element by assignment.
    var empty = [];          // [] is an empty array with no elements.
    empty.lenght             // => 0
 
    // Arrays and objects can hold other arrays and objects:
    var points = [             // An array with 2 elements.
        {x:0, y:0},            // Each element is an object
        {x:1, y:1}
    ];
    var data = {                 // An object with 2 properties
        trial1: [[1,2], [3,4]], // The value of each property is an array.
        trial2: [[2,3], [4,5]]  // The elements of the arrays are arrays.
    };
   

4.

    // Operators act on values (the operands) to produce a new value.
    // Arithmetic operators are the most common:
    3 + 2            // => 5: additon
    3 - 2 
    3 * 2
    3 / 2
    points[1].x - points[0].x      // => 1: more complicated operands work, too
    "3" + "2"                      // => "32": + adds numbers, concatenates strings
    
    // JavaScript defines some shorthand arithmetic operators
    var count = 0;              // Define a variable
    count++;                    // Increment the variable
    count--;                    // Decrement the variable
    count += 2;                 // Add 2: same as count = count + 2;
    count *= 3;                 // Multiply by 3: same as count = count * 3;
    count                       // => 6: variable names are expressions. too

    // Equality and relational operators test whether two values are equal, 
    // unequal, less than, greater than, and so on. They evaluate to true or false.
    var x = 2, y = 3;        // These = signs are assignment, not equality tests
    x == y                   // => false: equality
    x != y                   // => true: inequality
    x < y                    // => true: less-than
    x <= y
    x > y
    "two" == "three"      // => false: the two strings are different
    "two" > "three"       // => true: "tw" is alphabetically greater than "th"
    false == (x > y)      // => true: false is equal to false

    // Logical operators combine or invert boolean values
    (x == 2) && (y == 3)    // => true: both comparisons are true. && is AND
    (x > 3) || (y < 3)      // => false: neither comparison is true. || is or
    !(x == y)               // => true: ! inverts a boolean value
   

5.

    // Functions are parameterized blocks of JavaScript code that we can invoke.
    function plus1(x) { // Define a function named "plus1" with parameter "x"
        return x+1; // Return a value one larger than the value passed in
    } // Functions are enclosed in curly braces

    plus1(y) // => 4: y is 3, so this invocation returns 3+1

    var square = function(x) { // Functions are values and can be assigned to vars
        return x*x; // Compute the function's value
    }; // Semicolon marks the end of the assignment.
    square(plus1(y)) // => 16: invoke two functions in one expression