Use redefining helper pattern for better tree-shakeability

[Andarist]

• Code is up-to-date with the master branch
• You've successfully run jake runtests locally
• You've signed the CLA

I'm not yet sure if I've made this change correctly, need to fix tests first - but this presents the idea, so let me know if you'd like to proceed with this or not. By changing extendStatics & assign to declarations they become effectively more static and thus it makes the whole thing more recognizable as side-effect free, so other tooling has easier job at tree-shaking them.

We use this pattern currently in babel. You can check here.

This PR fails at:

TypeError: Assignment to constant variable.

I suspect that those helpers are transformed to const somewhere, but I don't know where yet. I'd appreciate some hint about this one

[msftclas]

All CLA requirements met.

[j-oliveras]

Your change is not valid. Running your __assign version with node 8.11.1 calling as __assign({}, {a: 1}) I get an RangeError: Maximum call stack size exceeded exception instead of { a: 1}.

[Andarist]

You are right, I've reassigned wrong identifier - it should be fixed now.

[mhegazy]

@rbuckton thoughts?

[rbuckton]

@mhegazy - These changes look fine to me.

@Andarist - Where are you seeing "TypeError: Assignment to constant variable."? Is it from reassigning extendsStatics since it is declared as a function?

[Andarist]

@Andarist - Where are you seeing "TypeError: Assignment to constant variable."? Is it from reassigning extendsStatics since it is declared as a function?

Not sure, but I was reassigning wrong identifier at first - I've fixed it and the problem is gone. However still the received error seemed to be wrong as my code error was causing runtime recursion error and not TypeError, but 🤷‍♂️

I've updated baseline tests, from my perspective it is ready to merge if you decide the change is worth it 😉

[j-oliveras]

I think that the @rbuckton comment is because your current __extends at playground has a compile error at extendStatics assignment with message Cannot assign to 'extendStatics' because it is not a variable. The code is:

var __extends = (this && this.__extends) || (function () {
	function extendStatics(d, b) {
		extendStatics = Object.setPrototypeOf ||
			({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
			function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
		return extendStatics(d, b);
	}

	return function (d, b) {
		extendStatics(d, b);
		function __() { this.constructor = d; }
		d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
	};
})();

Seems it is valid to reassign a function in javascript but typescript don't allow it. I don't know if is by design.

How to change it to this?

var __extends = (this && this.__extends) || (function () {
    var extendStatics = function (d, b) {
        extendStatics = Object.setPrototypeOf ||
            ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
            function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
        return extendStatics(d, b);
    };

    return function (d, b) {
        extendStatics(d, b);
        function __() { this.constructor = d; }
        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
    };
})();

[Andarist]

Sure, I can change extendStatics to function expression if needed - just let me know.

[Andarist]

@rbuckton friendly 🏓 is there anything left to do here? is this something that you would like to incorporate?

[rbuckton]

One minor change, per the earlier discussion.

Also, would you be interested in making this change in https://github.com/Microsoft/tslib as well?

[rbuckton]

Could you make this a var per #24244 (comment)?

[Andarist]

One minor change, per the earlier discussion.

Done.

Also, would you be interested in making this change in https://github.com/Microsoft/tslib as well?

That was my original intention/reason why I've started this discussion here 😉 Is tslib manually updated or is it done with some script?

[mhegazy]

@rbuckton can u take one more look?

[Andarist]

@rbuckton friendly 🏓 😉

[rbuckton]

Sorry for the delay.

Is tslib manually updated or is it done with some script?

tslib is manually updated currently.

[Andarist]

tslib is manually updated currently.

Ok, thanks for the info - gonna prepare a followup PR there.