Js, what is 'this' when set the static function from a Class to variable object

Question

class Obj {
  static log(){
    console.log(this)
  }
}

var test = Obj.log

Obj.log() // class Obj
test() // undefined

This is the code I run without 'use strict'. The result I don't understand. Why does the function test log undefined instead of the global object?

What did you expect it to return?

RobG
– RobG

2017-03-25 08:08:10 +00:00
Commented Mar 25, 2017 at 8:08 — RobG
– RobG, Commented Mar 25, 2017 at 8:08
The window object like the simple function.

李晓爽
– 李晓爽

2017-03-25 08:09:38 +00:00
Commented Mar 25, 2017 at 8:09 — 李晓爽
– 李晓爽, Commented Mar 25, 2017 at 8:09

T.J. Crowder · Accepted Answer · 2017-03-25 08:47:35Z

6

It's because class code is always strict code. From the spec

NOTE A class definition is always strict code.

Since class code is always strict, the method log is a strict function (so amongst other things, its internal [[ThisMode]] slot is set to strict), and when you call a strict function without setting this, this is set to undefined for the call.

The chain of operations in the spec that sets this up is that a direct call expression is evaluated by eventually calling the spec's abstract EvaluateDirectCall operation with thisValue set to undefined. That in turn calls Call passing along the thisValue, which calls [[Call]] on the function passing thisValue as thisArgument, which calls OrdinaryCallBindThis, which checks the function's [[ThisMode]] and, if it's strict, sets the thisValue to thisArgument directly without further logic (whereas if it weren't strict, it would replace null and undefined with the global object and do ToObject on any other values).

edited Mar 25, 2017 at 8:47

answered Mar 25, 2017 at 8:06

T.J. Crowder

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2 Comments

李晓爽 Over a year ago

OrdinaryCallBindThis ( F, calleeContext, thisArgument ). test.call(obj, list). In this situation, F means test, calleeContext means obj, thisArgument means list. Is this right?

T.J. Crowder Over a year ago

@李晓爽: No. F is test and thisArgument is obj. The calleeContext is the execution context created by [[Call]]'s call to PrepareForOrdinaryCall. (My link to [[Call]] above was wrong, btw, I've fixed it.)

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