此代码抛出错误.
try {
alert(hasOwnProperty('window'));
} catch(e) {
alert(e); // Type Error : can't convert undefined to object
}
但是这段代码不会引发错误.
try {
alert(this.hasOwnProperty('window')); // true (if on browser)
} catch(e) {
// through catch block
alert(e);
}
就我而言,func(arg)等于this.func(arg)是否this为Global Object.为什么会发生这样的事情？
我认为正在发生的是我们有严格模式代码和非严格模式代码的交互.事实上,当我挖出一个Firefox 3.6.15(不支持严格模式)的副本时,我没有使用我发布到你的问题的链接(它警告"真"两次)得到错误.
显示的代码显然是非严格的模式代码.但是hasOwnProperty浏览器的实现呢？我怀疑它是严格的,在支持严格模式的浏览器上.
当你说
func();
...浏览器所做的是func使用标准标识符解析查找,然后像调用它一样调用它:
func.call(undefined);
如果func是松散模式函数,那么在调用中func,this是全局对象.但是,如果func是严格模式函数,则this在调用内undefined.
相比之下,有了这个:
this.func();
...它再次查找func(这次通过使用原型链的属性解析)然后有效地执行此操作:
this.func.call(this);
在严格或松散模式下,这意味着this在函数内this.(当然,在全球范围内,this是全球目标.)
以下是使用我们可以看到的代码进行此交互的示例,而不是hasOwnProperty:
(function() {
"use strict";
window.strictFunction = function() {
display("strictFunction: this === window? " +
(this === window));
display("strictFunction: typeof this: " +
typeof this);
};
})();
strictFunction();
strictFunction.call(undefined);
正如您所看到的,除了定义strictFunction函数的位之外,这是松散的代码window.然后我们从松散的代码中调用该函数两次.结果是这样的:
strictFunction: this === window? false strictFunction: typeof this: undefined strictFunction: this === window? false strictFunction: typeof this: undefined
相反,如果我们使用松散函数执行此操作,结果是:
looseFunction: this === window? true looseFunction: typeof this: object looseFunction: this === window? true looseFunction: typeof this: object
<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Fun With Strict Interactions</title>
<style>
body {
font-family: sans-serif;
}
p {
margin: 0;
}
</style>
</head>
<body>
<script>
(function() {
"use strict";
window.strictFunction = function() {
display("strictFunction: this === window? " +
(this === window));
display("strictFunction: typeof this: " +
typeof this);
};
})();
(function() {
window.looseFunction = function() {
display("looseFunction: this === window? " +
(this === window));
display("looseFunction: typeof this: " +
typeof this);
};
})();
display("Direct call:");
strictFunction();
looseFunction();
display("<hr>Call with <code>.call(undefined)</code>:");
strictFunction.call(undefined);
looseFunction.call(undefined);
display("<hr>Call with <code>.call(window)</code>:");
strictFunction.call(window);
looseFunction.call(window);
function display(msg) {
var p = document.createElement('p');
p.innerHTML = String(msg);
document.body.appendChild(p);
}
</script>
</body>
</html>
输出(使用支持严格模式的JavaScript引擎):
Direct call: strictFunction: this === window? false strictFunction: typeof this: undefined looseFunction: this === window? true looseFunction: typeof this: object -- Call with .call(undefined): strictFunction: this === window? false strictFunction: typeof this: undefined looseFunction: this === window? true looseFunction: typeof this: object -- Call with .call(window): strictFunction: this === window? true strictFunction: typeof this: object looseFunction: this === window? true looseFunction: typeof this: object
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