Why doesn't JavaScript ES6 support multi-constructor classes?

Question

I want to write my Javascript class like below.

class Option {     constructor() {         this.autoLoad = false;     }      constructor(key, value) {         this[key] = value;     }      constructor(key, value, autoLoad) {         this[key] = value;         this.autoLoad = autoLoad || false;     } } 

I think it would be nice if we can write out class in this way. Expect to happen:

var option1 = new Option(); // option1 = {autoLoad: false} var option2 = new Option('foo', 'bar',); // option2 = {foo: 'bar'} var option3 = new Option('foo', 'bar', false); // option3 = {foo: 'bar', autoLoad: false} 

Answer 1

I want to write my Javascript class like below

You can't, in the same way you can't overload standard functions like that. What you can do is use the arguments object to query the number of arguments passed:

class Option {     constructor(key, value, autoLoad) {         // new Option()         if(!arguments.length) {             this.autoLoad = false;         }         // new Option(a, [b, [c]])         else {             this[key] = value;             this.autoLoad = autoLoad || false;         }     } } 

Babel REPL Example

Of course (with your updated example), you could take the approach that you don't care about the number of arguments, rather whether each individual value was passed, in which case you could so something like:

class Option {     constructor(key, value, autoLoad) {         if(!key) { // Could change this to a strict undefined check             this.autoLoad = false;             return;         }         this[key] = value;         this.autoLoad = autoLoad || false;     } }