ES6 (ECMAScript 2015) support as of 2018

Question

As a long time programmer just getting into JavaScript programming, I have the following questions that are still unclear despite having read many articles.

• Looking at the ES6 (ECMAScript 2015) support by browsers, I can see that the supporting level is much less than that of Node.js, so the question is,
• If both Node.js and browsers are using the modern V8 engine, why supporting level are so different?
• Looking at the ES6 support in Node.js, I can see really really few ES6 features are unsupported now. However, what exactly does the supported means in the chart? I.e.,
• Does it means even I write using the support ES6 features, I still need to use the Babel compiler to compile ES6 code to ES5 for Node.js to use it?
• For TypeScript ES6-style JavaScript code that runs for Node.js, they are still need to be transpiled into an ES5 compatible form, despite that Node.js almost cover all ES6 featues, right? I.e.,

for the following code,

class Animal {
    constructor(public name) { }
    move(meters) {
        console.log(this.name + " moved " + meters + "m.");
    }
}

class Snake extends Animal {
    move() {
        console.log("Slithering...");
        super.move(5);
    }
}

class Horse extends Animal {
    move() {
        console.log("Galloping...");
        super.move(45);
    }

var sam = new Snake("Sammy the Python")
var tom: Animal = new Horse("Tommy the Palomino")

sam.move()
tom.move(34)

Does it need to be transpiled into an ES5 compatible form to runs with Node.js or not?

Finally, any online site that I can try playing with TypeScript/ES6 code like above?

• I copy it to my chrome console, and got an error that I don't understand - Unexpected strict mode reserved word, and
• I tried it on http://www.typescriptlang.org/play/index.html, but the console output is not working there.

Please help. thx.

Answer 1

Looking at the ES6 (ECMAScript 2015) support by browsers, I can see that the supporting level is much less than that of Node.js, so the question is,

Many different browsers and many different Javascript engines in them, each with their own level of ES6 support. The latest version of node.js is generally pretty up-to-date on what the V8 engine supports. Many browsers have longer release cycles and may not be as current, but each is different and has their own release strategy and level of ES6 support.

If both Node.js and browsers are using the modern V8 engine, why supporting level are so different?

If you compare the latest release of node.js with the latest release of Chrome on Windows, you won't see much difference in support. The ES6 support chart you're looking at seems old to me. For example, Chrome has had support for the Set object for a long time, but your chart says false.

Looking at the ES6 support in Node.js, I can see really really few ES6 features are unsupported now. However, what exactly does the supported means in the chart? I.e.,

Supported means you can use the feature directly without a transpiler, but how accurate that is depends upon the source of the document claiming it. Some documents do extensive testing of all the various edge cases of a given feature. Others just look for general implementation. So if for example, you're looking at support for the Set object and it says "supported", then that is suppose to mean that you can just write plain Javascript that uses the Set object and it will just work. How accurate that document is depends upon the source of their data and the thoroughness of their testing.

Does it means even I write using the support ES6 features, I still need to use the Babel compiler to compile ES6 code to ES5 for Node.js to use it?

No. In a Javascript engine that supports a given feature in ES6, you can write ES6 code for that feature and directly run it in that Javascript engine. No transpiling is needed.

For TypeScript ES6-style JavaScript code that runs for Node.js, they are still need to be transpiled into an ES5 compatible form, despite that Node.js almost cover all ES6 featues, right? I.e.,

The class definitions you show are plain ES6 code. Those will work just fine as is in an ES6 capable Javascript engine.

If you write Typescript code, then you will have to transpile the TypeScript to Javascript because no Javascript engine (I know of) supports TypeScript directly. When transpiling form TypeScript to Javascript, you can usually specify whether you want the transpiler to generate ES5 compatible code (which will run in an ES5 engine or an ES6 engine) or ES6 compatible code (which will only run in an ES6 engine) depending upon what your target environment is capable of.

Does it need to be transpiled into an ES5 compatible form to runs with Node.js or not?

Your particular code appears to contain at least one TypeScript style variable declaration which would need to be transpiled. The rest looks like plain ES6 Javascript which should work in any ES6 engine without transpiling.

When I remove the TypeScript, fix some syntax errors in your code and implement the Animal constructor properly, then this code works fine in node.js v8.8.1 (which is what I currently have installed) and in Chrome 63.0.3239.132, Edge 41.16299.15.0 and Firefox 57.0.4 all on Windows 10:

    // Generic ES6 code

    class Animal {
        constructor(name) { 
            this.name = name;
        }
        move(meters) {
            console.log(this.name + " moved " + meters + "m.");
        }
    }
    
    class Snake extends Animal {
        move() {
            console.log("Slithering...");
            super.move(5);
        }
    }
    
    class Horse extends Animal {
        move() {
            console.log("Galloping...");
            super.move(45);
        }
    }
    
    var sam = new Snake("Sammy the Python");
    var tom = new Horse("Tommy the Palomino");
    
    sam.move();
    tom.move(34);

You can run this snippet yourself in any browser you desired to see the results (assuming the browser is modern enough to support a stack overflow snipppet). It works in all the current versions of browsers I have except IE 11.192.16299.0 (no surprise that IE doesn't support ES6).

I copy it to my chrome console, and got an error that I don't understand - Unexpected strict mode reserved word,

This happened to me when I tried to run your code in node.js until I removed the TypeScript from it so that it was just plain ES6. I think this particular error is caused by the public in this line:

constructor(public name) { }

since that is not part of the ES6 specification (it's apparently part of TypeScript).

It seems that there's one question you're dying to ask, but haven't exactly articulated is: "How do you know whether you have to transpile or not?".

The answer is that you have to understand the cross between the target environments you wish to run in and the newest features you plan to use. If you are writing server-side code that will only run in node.js, then it's a lot simpler. Examine a comprehensive table such as http://node.green/, study what it says for the node.js version you plan to use and the feature in question. If it indicates you should be able to use that feature, then write your code using that feature, write a test case for it and verify that both the code you wrote and the feature you are using both work. Add that to your body of knowledge about what you can and can't use in that version of node.js. You can then assume all future versions of node.js will also support that feature.

If you're writing code to run in a browser, life is much more complicated. If you plan to support a lot of browsers and really don't want to worry about ES6 support at all, then just transpile to an ES5 target and go about your business.

If you want to use non-transpiled code, then you have a lot of testing to do in a lot of browsers. You have to first specify exactly which versions of which browsers you are going to support and then you have to write your code and test cases and you have to test in every browser you plan to support. There really is no shortcut. When you find things that don't work, you'll have to either look for polyfills or work-arounds or stop using that ES6 feature.