JavaScript 全局属性和全局方法

JavaScript Global Properties 全局属性

• Infinity：
  • Infinity is a numeric value that represents positive infinity. 正无穷大
  • -Infinity is a numeric value that represents negative infinity. 负无穷大
  • Infinity is displayed when a number exceeds the upper limit of the floating point numbers, which is 1.797693134862315E+308.
  • -Infinity is displayed when a number exceeds the lower limit of the floating point numbers, which is -1.797693134862316E+308.
    示例：

console.log(2.797693134862315E+308) // 输出 Infinity
console.log(-2.797693134862315E+308) // 输出 -Infinity

• NaN："Not-a-Number" value 非数字
• undefined：Indicates that a variable has not been assigned a value 没有赋值的变量
  示例：

var x;

if (x === undefined) {
    txt = "x is undefined";
} else {
    txt = "x is defined";
}

JavaScript Global Functions 全局方法

encodeURI() & decodeURI() 编码/解码URI

示例：

var uri = "my test.asp?name=a&car=b";
var enc = encodeURI(uri); // my%20test.asp?name=a&car=b
var dec = decodeURI(enc); // my test.asp?name=a&car=b

eval()

• 执行一段 JavaScript 代码
• 计算一段 JavaScript 表达式
  示例：

var x = 10;
var y = 20;
var a = eval("x * y"); // 200
var b = eval("2 + 2"); // 4
var c = eval("x + 17"); // 27

isFinite()

• 判断值是不是有限的合法的数字
  示例：

var e = isFinite("123"); // true
var f = isFinite("Hello"); // false
var g = isFinite("2005/12/12"); // false

isNaN()

• 判断值是不是非法的数字
  示例：

isNaN('123') // false
isNaN('Hello') // true
isNaN('2005/12/12') // true

Number()

• 将值转换成数字
  示例：

var x1 = true;
var x2 = false;
var x3 = new Date();
var x4 = "999";
var x5 = "999 888";

Number(x1); // 1
Number(x2); // 0
Number(x3); // 1382704503079
Number(x4); // 999
Number(x5); // NaN

parseInt() & parseFloat()

• 将字符串转换成 Int 或 Float
  示例：

var a = parseInt("10"); // 10
var b = parseInt("10.00"); // 10
var d = parseInt("34 45 66"); // 34
var e = parseInt(" 60 "); // 60
var f = parseInt("40 years"); // 40
var g = parseInt("He was 40"); // NaN

String()

• 将值转换成字符串
  示例：

var x1 = Boolean(0);
var x2 = Boolean(1);
var x3 = new Date();
var x4 = "12345";
var x5 = 12345;

String(x1); // false
String(x2); // true
String(x3); // Fri Dec 01 2017 15:15:53 GMT+0800 (China Standard Time)
String(x4); // 12345
String(x5); // 12345

引用：
JavaScript Global Reference