Review (TL;DR)

Determining the this binding for an executing function requires finding the direct call-site of that function. Once examined, four rules can be applied to the call-site, in this order of precedence:

1. Called with new? Use the newly constructed object.

2. Called with call or apply (or bind)? Use the specified object.

3. Called with a context object owning the call? Use that context object.

4. Default: undefined in strict mode, global object otherwise.

Be careful of accidental/unintentional invoking of the default binding rule. In cases where you want to “safely” ignore a this binding, a “DMZ” object like ø = Object.create(null) is a good placeholder value that protects the global object from unintended side-effects.

Instead of the four standard binding rules, ES6 arrow-functions use lexical scoping for this binding, which means they adopt the this binding (whatever it is) from its enclosing function call. They are essentially a syntactic replacement of self = this in pre-ES6 coding.