<<High Performance JavaScript>>读书笔记-2.Data Access

Scope Chains and Identifier Resolution

function add(num1, num2){
var sum = num1 + num2;
return sum;
}

Scope Chain Augmentation
with表达式会临时改变execution context中的Scope chain,一个新的包含指定对像所有属性的可变对象被插入到execution
context的scopechain的最前端。这样local variable的访问就会变慢，所以不要使用with。

try {
methodThatMightCauseAnError();
} catch (ex){
alert(ex.message); //scope chain is augmented here
}

Dynamic Scopes

Both the with statement and the catch clause of a try-catch statement, as well as a function containing
eval(), are all considered to be dynamic scopes.

Closures, Scope, and Memory

function assignEvents(){
var id = "xdi9592";
document.getElementById("save-btn").onclick = function(event){
saveDocument(id);
};
}

通常Activationobject会同execution
context一起销毁，但是使用了closure后，Activation object仍然被closure引用，所以还不能销毁。
因此使用闭包的函数比未使用闭包的函数需要更多的内存



访问id和saveDocument会有性能下降

Object Members

function Book(title, publisher){
this.title = title;
this.publisher = publisher;
}
Book.prototype.sayTitle = function(){
alert(this.title);
};