Understanding "this" within anonymous function in JavaScript -

in post, lots of answers there discussing this keyword in javascript. however, still confuse this in anonymous function following

// mymodule.js 'use strict'; (function(handler) {     // export methods     handler.b = b;     handler.a = a;      function a() {         console.log(this);         console.log('function invoked...');     }      function b() {         console.log(this);         console.log('function b invoked...');         try {             a();             this.a();         } catch (err) {             console.log('exception ' + err);         }     } })(module.exports);  // test.js var mymodule = require('mymodule.js'); mymodule.b(); 

output: (running under node.js)

{ b: [function: b], a: [function: a] } function b invoked...  undefined function invoked...  { b: [function: b], a: [function: a] } function invoked... 

the output indicates function a in 2 different scopes. right? why there 2 scopes function a?

as know, this related scope. , this in anonymous function of mymodule undefined. according output, 1 of scope of function a undefined, other { b: [function: b], a: [function: a] }. difference between them?

this , scope have nothing each other. in javascript, this set how function called, not it's defined. (there 2 exceptions rule, i'll mention them below.)

so when you're calling a, you're setting this during call (largely implicitly). when this:

a(); 

...you're calling a without doing explicit set this should be; result, you're implicitly calling this set undefined, because code in strict mode. (if in loose mode, you'd calling this set reference global object.) it's worth noting you're resolving identifier a via context created call anonymous function, contains a , b (effectively) variables.

but here:

this.a(); 

...you're calling a part of expression getting function reference object property (a; note different meaning a, both property , context variable refer same function). act of doing calls a this set reference object got property from.

that's why see 2 different values this in a.

the exceptions "this set how call it" rule are:

1. es6's "arrow" functions, inherit this context (not scope) they're created.

2. es5's "bound" functions (the result of calling .bind on function reference), have this baked them .bind call , see same value this.