"this & object prototypes": cover strict mode `this` differences

[getify]

Apparently in ES6 strict mode, this values of primitives are no longer boxed. News to me!

function foo() {
   "use strict";
   console.log(typeof this);
}

foo.call("hello"); // "string"

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-ecmascript-function-objects

"...means that the this value is used exactly as provided by an invocation of the function"

(credit to @webreflection -- https://twitter.com/webreflection/status/522389313800400896)

[WebReflection]

thanks for the mention, probably worth mentioning that as soon as the outer scope runs in 'use strict' every callback is affected, included the this passed to Array extras.

(function(){
  'use strict';
  console.log([1, 2, 3].some(function(i) {
    return i === this;
  }, 2));
}());

Cheers

[RReverser]

Yes, the only place not affected is prototype (as methods still require boxing):

String.prototype.doThis = function () { console.log(typeof this) };
'abc'.doThis();

[WebReflection]

no @RReverser you forgot the 'use strict' directive, there's no exception to the rule.

String.prototype.doThis = function () { 'use strict'; console.log(typeof this) };
'abc'.doThis();

// string

[anba]

this binding for strict mode functions was already defined that way in ES5 (http://ecma-international.org/ecma-262/5.1/#sec-10.4.3 - step 1).

[getify]

facepalm

[WebReflection]

thanks @anba … I actually forgot it went in as part of ES5 spec but I am pretty sure some browser in between ( iOS5.1 or Android 2.2 ) didn't get that right.

Good to know that's a valid, modern, expectation for a context, it also makes sense since ES5 introduced this as undefined which is a different typeof too …. the takeaway: don't believe this is always an object, it can be anything ( so be aware … and use it when convenient )

Cheers to you all