操作对象的属性：分配方法来属性

Question

为什么这回空

var char = str.charAt(i); 
    char.isSpace = /\s/.test(char); 
    return char.isSpace; 

虽然这会返回一个有效的布尔？

var char = str.charAt(i); 
    var isSpace = /\s/.test(char); 
    return isSpace; 

Answer 1

var char = str.charAt(i); 
// after this assignment char is a string (but not String Object) 
char.isSpace = /\s/.test(char); 
// 1. you can't assign a property to char, it's not an Object 
// 2. if you could, you weren't assigning a method to char, but a value 
return char.isSpace; 
// so, char.isSpace is undefined 

您可以方法分配给String（静态）或String.prototype，如参见：

String.isWhitespace = function (str) {return /\s/g.test(str);} 
var char = str.charAt(i); 
String.isWhitespace(char); //=> true/false 

String.prototype.isWhitespace = function() {return /\s/g.test(str);} 
var char = str.charAt(i); 
char.isWhitespace(); //=> true/false