Javascript基础(2) - 严格模式特点

添加了保留字 protected，static 和 interface

在严格模式下，不可以用with()

(function(){
//非严格模式
var a = {name: "Bob"};
with(a){
name = "Lily";
}
console.log(a.name);  //Lily
})();

(function(){
"use strict";   //严格模式
var a = {name: "Bob"}, b = {};
with(a, b){    //SyntaxError: Strict mode code may not include a with statement
name = "Lily";
}
console.log(a.name);
})();

在严格模式下，变量必须显示声明(var/let/const)

(function(){
//非严格模式
a = 10;
console.log(a);  //10
})();

(function(){
"use strict";   //严格模式
b = 10;   //ReferenceError: b is not defined
console.log(b);
})();

在严格模式下，this默认是undefined

(function(){
//非严格模式
console.log(this);  //window
})();

(function(){
"use strict";   //严格模式
console.log(this);    //undefined
})();

在严格模式下，为只读变量和不可扩展对象赋值会报错, 而不是静默失败

(function(){
//非严格模式
var o = {
name: "Lily"
};
Object.freeze(o);
o.name = "Bob";
o.age = 20;
console.log(o);  //Object {name: "Bob"}
})();

(function(){
"use strict";   //严格模式
var o = {
name: "Lily"
};
Object.freeze(o);
o.name = "Bob";  //TypeError: Cannot assign to read only property 'name' of object '#<Object>'
o.age = 20;  //TypeError: Can't add property age, object is not extensible
console.log(o);
})();

在严格模式下，不可以在eval参数中定义变量和函数

(function(){
//非严格模式
var str1 = "var name='Lily';";
var str2 = "function fun1(){console.log('hello');}";
eval(str1);   //这个name定义在了全局，而不是函数内
eval(str2);
console.log(name);   //Lily
fun1();    //hello
})();

(function(){
"use strict";   //严格模式
var str1 = "var alias='Lily';";
var str2 = "function fun2(){console.log('hello');}";
eval(str1);
eval(str2);
eval("name = 'Bob'");    //修改全局变量name
console.log(name);   //Bob
console.log(alias);    //ReferenceError: alias is not defined
fun2();    //ReferenceError: fun is not defined
})();

在严格模式下，有名参数是arguments参数的静态副本，而非引用。

(function(){
//非严格模式
var name = "Bob";
test(name);

function test(alias){
alias = "Lily";
console.log(alias);  //Lily
console.log(arguments[0]);  //Lily
}
})();

(function(){
"use strict";   //严格模式
var name = "Bob";
test(name);

function test(alias){
alias = "Lily";
console.log(alias);  //Lily
console.log(arguments[0]);  //Bob
}
})();

在严格模式下，用delete删除var声明的变量和不可配置属性时抛出异常，而不是静默失败（返回false)

(function(){
//非严格模式
var a = 10;
var fun = function(){console.log("fun called");};
var o = Object.defineProperty({}, "name", {
value: "Bob"
});   //默认即不可配置
delete a;   //false
console.log(a);  //10

delete fun;   //false
fun();  //fun called

delete o.name;   //false
console.log(o.name);  //Bob

//删除一个不存在的变量
delete no;  //false

})();

(function(){
"use strict";   //严格模式
var a = 10;
var fun = function(){console.log("fun called");};
var o = Object.defineProperty({}, "name", {
value: "Bob"
});   //默认即不可配置
//delete a;   //SyntaxError: Delete of an unqualified identifier in strict mode.
console.log(a);

delete fun;  //SyntaxError: Delete of an unqualified identifier in strict mode.
fun();

delete o.name;  //SyntaxError: Delete of an unqualified identifier in strict mode.
console.log(o.name);

//删除一个不存在的变量
delete no;  //SyntaxError: Delete of an unqualified identifier in strict mode.

})();

在严格模式下，arguments和eval是关键字，不能被修改

(function(){
//非严格模式
eval = 10;
eval("console.log('hello');");  //TypeError: eval is not a function

(function(){
arguments = 20;
console.log(arguments);   //20
}());

})();

(function(){
"use strict";   //严格模式
eval = 10;  //SyntaxError: Unexpected eval or arguments in strict mode
eval("console.log('hello');");

(function(){
arguments =20;  //SyntaxError: Unexpected eval or arguments in strict mode
console.log(arguments);
}());

})();

在严格模式下，不可以用8进制

(function(){
//非严格模式
console.log(070);  //56 (因浏览器而异)
})();

(function(){
"use strict";   //严格模式
console.log(070);  //SyntaxError: Octal literals are not allowed in strict mode.
})();

在严格模式下，函数的形参不可以同名

(function(){
//非严格模式
var one = 1;
var two = 2;
fun(one, two);   //2
function fun(a,a){
console.log(a);
}
})();

(function(){
"use strict";   //严格模式
var one = 1;
var two = 2;
fun(one, two);
function fun(a,a){  //SyntaxError: Duplicate parameter name not allowed in this context
console.log(a);
}
})();

在严格模式下，不可以使用caller和arguments的属性，会报错

(function(){
//非严格模式
A();

function A(){
B();
}
function B(){
console.log(B.caller);   //function A(){ B(); }
console.log(arguments.callee);    //function B(){ console.log(B.caller); console.log(arguments.callee); }
}
})();

(function(){
"use strict";   //严格模式
A();

function A(){
B();
}
function B(){
console.log(B.caller);   //TypeError: 'caller' and 'arguments' are restricted function properties and cannot be accessed in this context.
console.log(arguments.callee);    //TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them
}