From Evernote:

Javascript Clipped from: http://jqfundamentals.com/chapter/javascript-basics

只有五种基本类型（primitives），其他都是对象

As it turns out, most everything we work with in JavaScript is an object — in fact, there are only five kinds of values that are not objects:

• strings (text) ： 'Aliz'
• booleans (true/false)
• numbers
• undefined
• null

对象声明和使用：

var person = {
firstName : 'Boaz',
lastName : 'Sender'
};

log( 'First name: ' + person.firstName ); // dot notation
log( 'Last name: ' + person[ 'lastName' ] ); // bracket notation，需加引号

 

函数命名有两种方法，只用第一种

var addTwoNumbers = function(a, b) {
return a + b;
};

 

function addTwoNumbers(a, b) {
return a + b;
}

Bottom line: naming functions using the function declaration approach can have unexpected results if you don't fully understand a feature of JavaScript known as hoisting. The details of hoisting are beyond the scope of this guide, but for now we'll stick to assigning function expressions to variables.

变量：

• you should almost always declare variables with a var statement
• variables declared inside of a function using a var statement are not available outside of that function
• variables declared without a var statement are always global

Variables that are declared inside a function with a var statement are only available inside of the function; this is generally desirable, and all variables should be declared with a var statement unless they are intended to be global — that is, available anywhere in your code. This usually isn't what you want, unless you want it to be possible for other code to change the value of the variable.

 

对象内的函数：

var person = {
firstName : 'Boaz',
lastName : 'Sender',
greet : function() {
log( 'Hi, ' + this.firstName );
}
};

person.greet(); // logs 'Hi, Boaz'

var sayIt = person.greet; // store the method in a variable

sayIt(); // logs 'Hi, undefined' -- uh-oh 因为 this 含义改变了

this. It refers to the object that is the context in which the function was called.

When we store the .greet() method in a variable sayIt and then call sayIt(), the context object changes to the global window object, not the person object. Since the window object doesn't have a property firstName, we getundefined when we try to access it.

What's a developer to do? First, be aware that storing object methods in variables can have unintended consequences for the meaning of this. Second, be aware that you can force the meaning of this to be what you want it to be, using the.call() or .apply() method on the function itself.

sayIt.call( person ); // logs 'Hi, Boaz'

 

Array 使用方法

var myArray = [ 'a', 'b', 'c' ];
var firstItem = myArray[ 0 ];

var secondItem = myArray[ 1 ]; // access the item at index 1
log( secondItem ); // logs 'b'

var len = myArray.length;

var i;

for (i = 0; i < len; i = i + 1) {
log( 'item at index ' + i + ' is ' + myArray[ i ] );
}

 

Logic operation:

五种 false

As it turns out, most values in JavaScript are truthy — in fact, there are only five values in JavaScript that are falsy:

• undefined (the default value of declared variables that are not assigned a value)
• null
• NaN ("not a number")：将非数学类型进行数学操作，会得到 NaN，如 'five' - '4'
• 0 (the number zero)
• '' (an empty string)

It's important to remember that all other values aside from the five values listed above are truthy. This includes empty arrays, empty objects, all non-empty strings (including the string '0'), and all numbers other than 0.

逻辑与 (&&) and 逻辑或 (||) operations.

var propertyName = ( dim === 'width' ) ? 'clientWidth' : 'clientHeight';

相等操作 === 三个等号