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javascriptbabeljstail-recursion

does Babel with the `es2016` preset implement tail call optimization?

I used the following example to test tail call recursion with Babel and the es2016 preset:

'use strict';

try {
    function r(n) {
        if (n%5000===0)
            console.log(`reached a depth of ${n}`);
        r(n+1);
    }
    r(0);
} catch (e) {
    if (!(e instanceof RangeError))
        throw e;
    else
        console.log('stack blown');
}

My package.json file is:

{
    "name": "tail-call-optimization",
    "version": "1.0.0",
    "description": "",
    "main": "index.js",
    "scripts": {
        "build": "babel es6 --out-dir es5 --source-maps",
        "watch": "babel es6 --out-dir es5 --source-maps --watch",
        "start": "node es5/app.js"
    },
    "author": "",
    "license": "ISC",
    "devDependencies": {
        "babel-cli": "^6.6.5",
        "babel-core": "^6.7.4",
        "babel-loader": "^6.2.4",
        "babel-polyfill": "^6.7.4",
        "babel-preset-es2016": "^6.0.10",
        "babel-runtime": "^6.6.1"
    },
    "dependencies": {
        "babel-polyfill": "^6.7.4",
        "source-map-support": "^0.4.0"
    }
}

... and the .babelrc is simply:

{
    "presets": ["es2016"]
}

Running the above with:

npm run build && npm run start

... results in the following console output:

reached a depth of 0
reached a depth of 5000
reached a depth of 10000
reached a depth of 15000
stack blown

Indeed, looking at the transpiled file in the es5 directory, there's nothing to suggest that TCO has been implemented.

Am I missing something?

My node version is 4.3.2.

Solution

  • None of the "official" Babel 6 plugins / presets currently implements TCO. babel-preset-es2016 is not an "official" preset. Unless TCO relies on parser support in Babylon (off the top of my head I wouldn't think so, but I'm not sure) then I suppose a userland plugin / preset could implement it, and perhaps does (but not that I know of). Here is the issue tracking eventual "official" re-implementation: T2614. If someone wants to PR that link into the Learn ES2015 docs @Marcus mentioned ping me here and I'll merge it.