CHAPTER 4 Expressions and Operators

4.1 Primary Expressions

1.23 // A number literal
"hello" // A string literal
/pattern/ // A regular expression literal
true // Evalutes to the boolean true value
false // Evaluates to the boolean false value
null // Evaluates to the null value
this // Evaluates to the "current" object
i // Evaluates to the value of the variable i.
sum // Evaluates to the value of the variable sum.
undefined // undefined is a global variable, not a keyword like null.

4.2 Object and Array Initializers

[] // An empty array: no expressions inside brackets means no elements
[1+2,3+4] // A 2-element array. First element is 3, second is 7
var matrix = [[1,2,3], [4,5,6], [7,8,9]];
var sparseArray = [1,,,,5];

Object initializer expressions

var p = { x:2.3, y:-1.2 }; // An object with 2 properties
var q = {}; // An empty object with no properties
q.x = 2.3; q.y = -1.2; // Now q has the same properties as p

object字面量可以被嵌套：

var rectangle = { upperLeft: { x: 2, y: 2 },
lowerRight: { x: 4, y: 5 } };

var side = 1;
var square = { "upperLeft": { x: p.x, y: p.y },
'lowerRight': { x: p.x + side, y: p.y + side}};


4.4 Property Access Expressions

expression . identifier
expression [ expression ]

var o = {x:1,y:{z:3}}; // An example object
var a = [o,4,[5,6]]; // An example array that contains the object
o.x // => 1: property x of expression o
o.y.z // => 3: property z of expression o.y
o["x"] // => 1: property x of object o
a[1] // => 4: element at index 1 of expression a
a[2]["1"] // => 6: element at index 1 of expression a[2]
a[0].x // => 1: property x of expression a[0]

4.5 Invocation Expressions

f(0) // f is the function expression; 0 is the argument expression.
Math.max(x,y,z) // Math.max is the function; x, y and z are the arguments.
a.sort() // a.sort is the function; there are no arguments.

4.6 Object Creation Expressions

new Object()
new Point(2,3)

如果没有参数的话：

new Object
new Date

4.8.1 The + Operator

如果其中有一个是object，那么就将它变成原始变量，Date object调用toString，其他的object调用valueOf函数，多数不含有有效的valueOf函数，所以调用toString。

如果有任何一个变成了string,那么另一个也变成String来进行运算。

否则都变成number，来进行运算。

1 + 2 // => 3: addition
"1" + "2" // => "12": concatenation
"1" + 2 // => "12": concatenation after number-to-string
1 + {} // => "1[object Object]": concatenation after object-to-string
true + true // => 2: addition after boolean-to-number
2 + null // => 2: addition after null converts to 0
2 + undefined // => NaN: addition after undefined converts to NaN

1 + 2 + " blind mice"; // => "3 blind mice"
1 + (2 + " blind mice"); // => "12 blind mice"

1 + 2 // Addition. Result is 3.
"1" + "2" // Concatenation. Result is "12".
"1" + 2 // Concatenation. 2 is converted to "2". Result is "12".
11 < 3 // Numeric comparison. Result is false.
"11" < "3" // String comparison. Result is true.
"11" < 3 // Numeric comparison. "11" converted to 11. Result is false.
"one" < 3 // Numeric comparison. "one" converted to NaN. Result is false.

4.9.3 The in Operator

var point = { x:1, y:1 }; // Define an object
"x" in point // => true: object has property named "x"
"z" in point // => false: object has no "z" property.
"toString" in point // => true: object inherits toString method
var data = [7,8,9]; // An array with elements 0, 1, and 2
"0" in data // => true: array has an element "0"
1 in data // => true: numbers are converted to strings
3 in data // => false: no element 3

4.9.4 The instanceof Operator

var d = new Date(); // Create a new object with the Date() constructor
d instanceof Date; // Evaluates to true; d was created with Date()
d instanceof Object; // Evaluates to true; all objects are instances of Object
d instanceof Number; // Evaluates to false; d is not a Number object
var a = [1, 2, 3]; // Create an array with array literal syntax
a instanceof Array; // Evaluates to true; a is an array
a instanceof Object; // Evaluates to true; all arrays are objects
a instanceof RegExp; // Evaluates to false; arrays are not regular expressions

4.12 Evaluation Expressions

javascript可以解释String到javascript代码，评价出它们的值，它使用全局范围的eval函数，如
eval("3+2") // => 5

eval里面如果使用x=1的语句，那么x这个变量的值将会被改变

4.13.3 The delete Operator

var o = { x: 1, y: 2}; // Start with an object
delete o.x; // Delete one of its properties
"x" in o // => false: the property does not exist anymore
var a = [1,2,3]; // Start with an array
delete a[2]; // Delete the last element of the array
a.length // => 2: array only has two elements now

var o = {x:1, y:2}; // Define a variable; initialize it to an object
delete o.x; // Delete one of the object properties; returns true
typeof o.x; // Property does not exist; returns "undefined"
delete o.x; // Delete a nonexistent property; returns true
delete o; // Can't delete a declared variable; returns false.
// Would raise an exception in strict mode.
delete 1; // Argument is not an lvalue: returns true
this.x = 1; // Define a property of the a global object without var
delete x; // Try to delete it: returns true in non-strict mode
// Exception in strict mode. Use 'delete this.x' instead
x; // Runtime error: x is not defined