admin 管理员组

文章数量: 1086455

 
var f = function(o){ return this+":"+o+"::"+(typeof this)+":"+(typeof o) };
f.call( "2", "2" );
// "2:2::object:string"

var f = function(o){ return this+":"+(typeof this)+":"+(typeof o); };
var x = [1,/foo/,"bar",function(){},true,[],{}];
for (var i=0;i<x.length;++i) console.log(f.call(x[i],x[i]));
// "1:object:number"
// "/foo/:object:object"
// "bar:object:string"
// "function () {\n}:function:function"
// "true:object:boolean"
// ":object:object"
// "[object Object]:object:object"

I see the same results in Chrome, Firefox, and Safari, so I assume it's per the spec, but...why? And where in the spec is this defined? And why not for functions?

var f = function(o){ return this+":"+o+"::"+(typeof this)+":"+(typeof o) };
f.call( "2", "2" );
// "2:2::object:string"

var f = function(o){ return this+":"+(typeof this)+":"+(typeof o); };
var x = [1,/foo/,"bar",function(){},true,[],{}];
for (var i=0;i<x.length;++i) console.log(f.call(x[i],x[i]));
// "1:object:number"
// "/foo/:object:object"
// "bar:object:string"
// "function () {\n}:function:function"
// "true:object:boolean"
// ":object:object"
// "[object Object]:object:object"

I see the same results in Chrome, Firefox, and Safari, so I assume it's per the spec, but...why? And where in the spec is this defined? And why not for functions?

  • javascript
  • call
  • ecma262
Share Improve this question edited Dec 8, 2010 at 21:35 Phrogz asked Dec 8, 2010 at 17:54 PhrogzPhrogz 304k113 gold badges667 silver badges757 bronze badges 1
  • wow, never knew that. now I wonder why that is as well! – Matt Commented Dec 8, 2010 at 18:03
Add a ment  | 

1 Answer 1

Reset to default 11

As defined in ECMA-262 ECMAScript Language Specification 3rd edition (see footnote), It's based on the spec (Section 15.3.4.4):

var result = fun.call(thisArg[, arg1[, arg2[, ...]]]);

Parameters

thisArg

Determines the value of this inside fun. If thisArg is null or undefined, this will be the global object. Otherwise, this will be equal to Object(thisArg) (which is thisArg if thisArg is already an object, or a String, Boolean, or Number if thisArg is a primitive value of the corresponding type). Therefore, it is always true that typeof this == "object" when the function executes.

Note in particular the last line.

The crucial thing is that js primitives (string, number, boolean, null, undefined) are immutable, so a function can not be attached to them. Therefore the call function wraps the primitive in an Object so that the function can be attached.

E.g.:

Doesn't work:

var test = "string";
//the next 2 lines are invalid, as `test` is a primitive 
test.someFun = function () { alert(this); }; 
test.someFun();

Works: 

var test = "string";
//wrap test up to give it a mutable wrapper
var temp = Object(test);
temp.someFun = function () { alert(this); };
temp.someFun();

(footnote) - as patrick dw noted in the ments, this will change in ECMA-262 ECMAScript Language Specification 5th edition when in strict mode:

From Section 15.3.4.4:

NOTE The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

本文标签： javascriptWhy does typeof this return quotobjectquotStack Overflow

版权声明：本文标题：javascript - Why does `typeof this` return &quot;object&quot;? - Stack Overflow 内容由网友自发贡献，该文观点仅代表作者本人， 转载请联系作者并注明出处：http://www.roclinux.cn/p/1744075964a2529352.html， 本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌抄袭侵权/违法违规的内容，一经查实，本站将立刻删除。