JavaScript syntax, data types

Case

Variable, function name, operator case-sensitive

Identifier

Variables, functions, attribute names, function parameters, the first character must be a character, an underscore (_), or dollar sign ($)
Other characters can be letters, underscores, dollar signs or numbers

Note

// 单行注释

/*
 * 多行注释
 *
 */

Strict Mode

ECMAScript 5 strict mode to introduce the concept to define a different JavaScript parsing and execution model

In strict mode, some uncertainties in the behavior of ECMAScript 3 are addressed, but also for some unsafe operations will throw an error.

Enable strict mode throughout the script, add at the top

"use stricr"

You can also specify a function to perform in strict mode

function doSomething() {
    "use stricr";
    // 函数体
}

Statement

Not forced to use a semicolon as the end of the statement, but do not recommend omitting title

var sum = a + b         // 没有分页也是有效语句,不推荐

var diff = a - b;       // 有效语句,推荐

Recommended use {}braces to combine a plurality of statements to the code block

if (test)
    alert(test);        // 有效但容易出错,不要使用

if (test) {             // 推荐使用
    alert(test);
}

Keywords and reserved words

Reference ECMA-262

variable

ECMAScript variable support loosely bound

Definition of variables varoperator, e.g.

var message;

var sum = 10;

var str = "hi";

Note: varthe variable will be the operator of the local variables defined in the definition of the variable scope

function test() {
    var message = "hi"
}
test();
console.log(message); // 错误

Although not varoperator can define global variables, but does not recommend this practice, because it is difficult to maintain, resulting in confusing code

function test() {
    message = "hi"
}
test();
console.log(message); // "hi"

You can use a statement to define a number of variables

var message = "hi",
    str = "hello",
    age = 29;

type of data

5 kinds of simple data types (basic data types)

Undefined
Null
Boolean
Number
String

One kind of complex data types

Object

`typeof`Operators

Use of a value typeofoperator may return one of the following string

UndefinedIf this value is not defined
booleanIf this value is a Boolean value
stringIf this value is a string
number - If this value is the value
objectIf this value is null or an object
function - If this value is a function of

var message = "some string";
console.log(typeof message);     // string
console.log(typeof (message));   // string
console.log(typeof 95);          // number

function f() {
    // do something
}
console.log(typeof f); // function

var result = true;
console.log(typeof result); // boolean

var value;
console.log(typeof value); // undefined

Note: typeofit is an operator, not a function of the code more kinds may be used parentheses, but not necessary

Undefined type

Undefined only one type of undefinedspecial value in use varwhen you declare a variable to be initialized but it, this variable is undefined is

var message;
console.log(message == undefined); // true

var message = undefined;    // 没有必要显式地将一个变量的值设置为undefined
console.log(message == undefined); // true

Null Type

Null only one type of nullspecial value, null value representing a null object pointer

var car = null;
console.log(typeof car); // "object"

Although the following code output true

console.log(null == undefined); // true

There is no need to explicitly set the value of a variable is undefined, but this rule does not apply to null, as long as the variable is intended to hold the object has not really save the object, you should definitely make the variable holds the null value,

Number Type

Number type represents the integer and floating point

It can be expressed in decimal, octal, hexadecimal numbers

Octal literal must first bit is 0, then the sequence of octal digits 0 to 7. If the value is out of range thereof literal, then the previous 0 are ignored, as the number after the decimal value analysis. Octal number in strict mode is invalid, and will throw an error

var num = 070;  // 八进制的56
var n = 079;    // 无效的八进制数字,解析为79

The first two hexadecimal literals must be 0x, followed by any hexadecimal numbers (0 to 9, and A ~ F), A ~ F case insensitive

var a = 0xa;    // 十六进制的10
var b = 0x1F;   // 十六进制的31

String type

string type is used to represent zero or more 16-bit Unicode character sequences of characters, i.e., the string may be represented by a double quote ( ") or a single quote ( ')

var str = "Hello, Wrold!";
var str = '你好, 世界!';

Characterized string length immutable

Converted to a string, using the toString()method

var a = 11;
var str = a.toString();
console.log(a + 1)      // 12
console.log(str + 1);   // 111

You can use toString()methods hex conversion

var num = 10;
console.log(num.toString());       //10
console.log(num.toString(2));      //1010
console.log(num.toString(8));      //12
console.log(num.toString(10));     //10
console.log(num.toString(16));     //a

Object Types

A collection of data and functionality

var o = new Object();
var a = new Object;     // 有效,但不建议省略圆括号

Each Object instance has the following attributes and methods

constructor Save the function used to create the current object
hasOwnProperty(propertyName) Checking for a given attribute in the current instance of the presence or absence of species
isPropertyOf(propertyName) Whether the object is a prototype for checking incoming current object
propertyIsEnumerablr(propertyName) It used to check whether a given attribute can use for-in statement to enumerate
toLocaleString() Returns a string representation of the object, the region corresponding to the execution environment of the string
toString() Returns a string representation of the object
valueOf() Returns the string object