nodejs error on es6 class constructor with default parameter value

Question

I am run simple es6 class code as the following:

'use strict';
class Polygon {
  constructor(height=44, width=55) { //class constructor
    this.name = 'Polygon';
    this.height = height;
    this.width = width;
  }

  sayName() { //class method
    console.log('Hi, I am a', this.name + '.');
   }
}

class Square extends Polygon {
  constructor(length) {
    super(length, length); //call the parent method with super
    this.name = 'Square';
  }

  get area() { //calculated attribute getter
    return this.height * this.width;
  }
}

let s = new Square();

 s.sayName();
 console.log(s.area);

It is running ok on chrome console. But it is running errors on nodejs(4.x, 5.x) as the following:

constructor(height=44, width=55) { //class constructor
                      ^

  SyntaxError: Unexpected token =
  at exports.runInThisContext (vm.js:53:16)
  at Module._compile (module.js:387:25)
  at Object.Module._extensions..js (module.js:422:10)
  at Module.load (module.js:357:32)
  at Function.Module._load (module.js:314:12)
  at Function.Module.runMain (module.js:447:10)
  at startup (node.js:148:18)
  at node.js:405:3

I think es6 do support default parameters for function, and chrome and node.js are run V8 engine, why do give out diff answer,...

hassansin · Accepted Answer · 2016-04-24 23:51:40Z

3

This is an in progress feature in 5.x which can be activated by the flag --harmony_default_parameters:

$ node -v 
v5.0.0
$ node --harmony_default_parameters  script.js

To see a list of in progress flags in your node version:

node --v8-options | grep "in progress"

answered Apr 24, 2016 at 23:51

hassansin

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2 Comments

changyh Over a year ago

hassansin: Thanks, It works,...But it is only on v5.x, and it is not long stable version, production can't use it, 4.x doesn't have this option,...

hassansin Over a year ago

Yup, not all version have this flag. Depends on which v8 engine it's using.

user1341217 · Accepted Answer · 2016-06-11 22:30:18Z

0

You can use Babel to transpile your code like this:

npm init
npm install --save-dev babel-cli babel-preset-es2015 babel-preset-stage-2
modify package.json so that it will contain the following script:

{ "scripts": { "start": "babel-node script.js --presets es2015,stage-2" } }
execute the script npm run start. It will output Hi, I am a Square. 2420

answered Jun 11, 2016 at 22:30

user1341217

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