在考虑向 String 原型(prototype)添加 trim 函数时,我遇到了一些对 JavaScript 字符串来说似乎很奇怪的事情。
if (typeof console === 'undefined') {
var console = { };
console.log = function(msg) {
alert(msg)
}
}
function isString(str) {
return ((str && typeof str === 'string') ||
(str && (str.constructor == String && (str.toString() !== 'null' && str.toString() !== 'undefined'))));
}
if (!String.prototype.trim) {
String.prototype.trim = function () {
return this.replace(/^\s*(\S*(?:\s+\S+)*)\s*$/, "$1");
};
}
function testing (str) {
if (isString(str)) {
console.log("Trimmed: " + str.trim() + " Length: " + str.trim().length);
} else {
console.log("Type of: " + typeof str);
}
return false;
}
function testSuite() {
testing(undefined);
testing(null);
testing("\t\r\n");
testing(" 90909090");
testing("lkkljlkjlkj ");
testing(" 12345 ");
testing("lkjfsdaljkdfsalkjdfs");
testing(new String(undefined)); //Why does this create a string with value 'undefined'
testing(new String(null)); //Why does this create a string with value 'null'
testing(new String("\t\r\n"));
testing(new String(" 90909090"));
testing(new String("lkkljlkjlkj "));
testing(new String(" 12345 "));
testing(new String("lkjfsdaljkdfsalkjdfs"));
}
现在我知道我们不应该使用 new 运算符创建字符串,但我不希望有人在未定义或空字符串上调用它,这些字符串更多是按照以下方式创建的:
new String ( someUndefinedOrNullVar );
我错过了什么?或者 !== 'null' && !== 'undefined' 检查是否真的有必要(删除该检查,将显示 'null' 和 'undefined')?
最佳答案
来自ECMA standard :
9.8 ToString
The abstract operation ToString converts its argument to a value of type String according to Table 13
[ the table shows undefined converts to "undefined" and null to "null"]
...然后:
15.5.2.1 new String ( [ value ] )
The [[Prototype]] internal property of the newly constructed object is set to the standard built-in String prototype object that is the initial value of String.prototype (15.5.3.1).
The [[Class]] internal property of the newly constructed object is set to "String".
The [[Extensible]] internal property of the newly constructed object is set to true.
The [[PrimitiveValue]] internal property of the newly constructed object is set to ToString(value), or to the empty String if value is not supplied.
因此,由于 ToString(undefined)
给出了 'undefined'
,所以这是有道理的。
关于JavaScript 是 item 一个字符串,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/3630477/