将RegExp.test分配给一个变量 [英] Assigning RegExp.test to a variable
问题描述
以下代码:
var r = /^[0-9A-Z]$/.test;
r(A)
引发'TypeError:无法将undefined转换为对象'
如何将测试函数分配给变量,用于传递函数,稍后评估等? (没有在另一个函数中包装正则表达式)
更新:
在回答之前考虑这一点有效的代码:
var o = {f:function(){return 1}};
var a = o.f;
var b = a(); // b = 1
在 test
方法中 this
的值。
例如:
pre $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $方法:function(){return this === obj}
};
obj.method(); // true
var method = obj.method;
method(); // false
如果您调用 test
方法作为一个函数 - 作为你的例子 r();
- , this
值将引用未定义
(对于ECMAScript 5中的内置函数或严格函数,在上面的示例中, this
将引用全局对象)。
调用 RegExp.prototype
的任何方法并使用这个
不是 RegExp
对象的值,将始终生成这个 TypeError
异常,引用规范:
在以下对RegExp原型对象属性的函数描述中,短语此RegExp对象指的是这个值用于调用该函数;如果此值不是对象或[[Class]]内部属性的值不是RegExp的对象,则会抛出TypeError异常。
然而,您可以将 test
方法绑定到 r
函数,使用 Function.prototype.bind
方法:
var re = / ^ [0- 9A-Z] $ /,
r = re.test.bind(re);
r(A); // true
或者使用调用
或 apply
:
var r = re.test;
r.call(re,A); // true
The following code:
var r = /^[0-9A-Z]$/.test;
r("A")
Throws 'TypeError: can't convert undefined to object'
How else could I assign the test function to a variable, for passing in functions, later evaluation, etc.? (Without wrapping the regex in another function)
Update:
Consider this bit of valid code before answering:
var o = { f: function() { return 1 } };
var a = o.f;
var b = a(); // b = 1
It has to do with the value of this
inside the test
method.
For example:
var obj = {
method: function () { return this === obj }
};
obj.method(); // true
var method = obj.method;
method(); // false
If you call the test
method "as a function" -as your example r();
-, the this
value will refer to undefined
(for built-in or strict functions in ECMAScript 5, in the above example this
will refer to the global object).
Calling any method of RegExp.prototype
with a this
value that is not a RegExp
object, will always generate this TypeError
exception, quoting the spec:
15.10.6 Properties of the RegExp Prototype Object
In the following descriptions of functions that are properties of the RegExp prototype object, the phrase "this RegExp object" refers to the object that is the this value for the invocation of the function; a TypeError exception is thrown if the this value is not an object or an object for which the value of the [[Class]] internal property is not "RegExp".
However you could bind the test
method to your r
function, using the Function.prototype.bind
method:
var re = /^[0-9A-Z]$/,
r = re.test.bind(re);
r("A"); // true
Or using call
or apply
:
var r = re.test;
r.call(re, "A"); // true
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