ESM build targets ES5 resulting in polyfills for await

[Flarna]

• This only affects the JavaScript OpenTelemetry library
• This may affect other libraries, but I would like to get opinions here first

The ESM typescript build targets ES5 currently for everything.
see

opentelemetry-js/tsconfig.base.esm.json

Line 2 in 07b19ad

"extends": "./tsconfig.es5.json",

and

opentelemetry-js/tsconfig.es5.json

Line 4 in 07b19ad

"target": "es5"

As a result typescript generates quite heavy polyfills for e.g. await like this extracted from transpiled Meter.ts file:

Is this intended? In special for NodeJs users this doesn't make much sense as ESM is only supported by newer NodeJs versions which support await out of the box.

The current ESM support was done for browsers as far as I remember. Are there any browsers supporting ESM but not await?

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
    function adopt(value) { return value instanceof P ? value : new P(function (resolve) { resolve(value); }); }
    return new (P || (P = Promise))(function (resolve, reject) {
        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
        function step(result) { result.done ? resolve(result.value) : adopt(result.value).then(fulfilled, rejected); }
        step((generator = generator.apply(thisArg, _arguments || [])).next());
    });
};
var __generator = (this && this.__generator) || function (thisArg, body) {
    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
    function verb(n) { return function (v) { return step([n, v]); }; }
    function step(op) {
        if (f) throw new TypeError("Generator is already executing.");
        while (_) try {
            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
            if (y = 0, t) op = [op[0] & 2, t.value];
            switch (op[0]) {
                case 0: case 1: t = op; break;
                case 4: _.label++; return { value: op[1], done: false };
                case 5: _.label++; y = op[1]; op = [0]; continue;
                case 7: op = _.ops.pop(); _.trys.pop(); continue;
                default:
                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
                    if (t[2]) _.ops.pop();
                    _.trys.pop(); continue;
            }
            op = body.call(thisArg, _);
        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
    }
};

[legendecas]

ESM is available since ECMAScript 2015 (aka. es5 edit: es6, sorry). So it doesn't make sense to targeting es5 when building esm targets.

[Flarna]

as far as I remember ECMAScript 2015 is es6.
to avoid the await polyfills completly target ES2017 is needed but using target ES2015 (or ES6) already removes quite some of it

[legendecas]

as far as I remember ECMAScript 2015 is es6.

@Flarna sorry, a typo.

tsconfig.base.json is already targeting es2017. That's default language version that we are publishing.

A naive investigation on feature matrix of modern browsers:

_	await	import statement
Chrome	55	61
Firefox	52	60
Edge	14	16

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await#browser_compatibility
• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#browser_compatibility

So I'm a bit confident that we can safely switch to tsconfig.base.json for tsconfig.esm.json's base config.

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[github-actions]

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