Typescript: Using --downlevelIteration causes complex plain array iteration emit

[athasach]

TypeScript Version: 2.7.0-dev.20171110

When using the --downlevelIteration compiler option, a plain JS array for..of iteration is transpiled into very complex code.

Code

let arr = [1, 2, 3];
for (const value of arr) {
    console.log(value);
}

Expected behavior:

Should emit:

"use strict";
var arr = [1, 2, 3];
for (var _i = 0, arr_1 = arr; _i < arr_1.length; _i++) {
    var value = arr_1[_i];
    console.log(value);
}

This above is emitted if --downlevelIteration is false.

Actual behavior:

With --downlevelIteration set to true, the following is emitted:

"use strict";
var __values = (this && this.__values) || function (o) {
    var m = typeof Symbol === "function" && o[Symbol.iterator], i = 0;
    if (m) return m.call(o);
    return {
        next: function () {
            if (o && i >= o.length) o = void 0;
            return { value: o && o[i++], done: !o };
        }
    };
};
var arr = [1, 2, 3];
try {
    for (var arr_1 = __values(arr), arr_1_1 = arr_1.next(); !arr_1_1.done; arr_1_1 = arr_1.next()) {
        var value = arr_1_1.value;
        console.log(value);
    }
}
catch (e_1_1) { e_1 = { error: e_1_1 }; }
finally {
    try {
        if (arr_1_1 && !arr_1_1.done && (_a = arr_1.return)) _a.call(arr_1);
    }
    finally { if (e_1) throw e_1.error; }
}
var e_1, _a;

This will be slower in hot code, but it also seems unnecessary if we know that this is a plain array.

[RyanCavanaugh]

We don't change the emitted code based on the types involved - see https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals non-goal number 5.

[allisoniskander]

@RyanCavanaugh Is there a distinction between changing emitted code based on type for a feature vs. for optimization?

[RyanCavanaugh]

No

[athasach]

Closing since this issue has been indicated to be working as intended.

[johan]

There is an interesting clash between non-goals 5 and 2, "Aggressively optimize the runtime performance of programs. Instead, emit idiomatic JavaScript code that plays well with the performance characteristics of runtime platforms." – where the here requested emitted code is very idiomatic (and also happens to be aggressively performant), and the currently emitted code is everything but either.

[NN---]

Unfortunately this decision has a huge performance impact in IE.
I have to choose between using ES5 output with fast array iteration or manually write this code in ES6 output and disallow for..of for arrays.
Please reconsider this.