Question

41

How do I make a public static field in an ES6 class?

rated 0 times [ 45] [ 4] / answers: 1 / hits: 79594 / 10 Years ago, wed, february 11, 2015, 12:00:00

I'm making a Javascript class and I'd like to have a public static field like in Java. This is the relevant code:

export default class Agent {

    CIRCLE: 1,

    SQUARE: 2,

    ...

This is the error I get:

line 2, col 11, Class properties must be methods. Expected '(' but instead saw ':'.

It looks like ES6 modules don't allow this. Is there a way to get the desired behavior or do I have to write a getter?

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

lizet

Add To Favorites

Follow

Total Points: 479

Total Questions: 96

Total Answers: 113

Location: Mexico

Member since Sun, Jul 25, 2021

3 Years ago

answered 10 Years ago jarrettw · Accepted Answer

You make public static field using accessor and a static keyword:

class Agent {

    static get CIRCLE() {

      return 1;

    }

    static get SQUARE() {

      return 2;

    }

}



Agent.CIRCLE; // 1

Looking at a spec, 14.5 — Class Definitions — you'd see something suspiciously relevant :)

ClassElement[Yield] :

MethodDefinition[?Yield]

static MethodDefinition[?Yield] ;

So from there you can follow to 14.5.14 — Runtime Semantics: ClassDefinitionEvaluation — to double check if it really does what it looks like it does. Specifically, step 20:

For each ClassElement m in order from methods

If IsStatic of m is false, then

Let status be the result of performing PropertyDefinitionEvaluation for m with arguments proto and false.

Else,

Let status be the result of performing PropertyDefinitionEvaluation for m with arguments F and false.

If status is an abrupt completion, then

Set the running execution context’s LexicalEnvironment to lex.

Return status.

IsStatic is defined earlier in 14.5.9

ClassElement : static MethodDefinition

Return true.

So PropertyMethodDefinition is called with F (constructor, function object) as an argument, which in its turn creates an accessor method on that object.

This already works in at least IETP (tech preview), as well as 6to5 and Traceur compilers.