How can we define class variables in ES6 without native support?

ES6 Class Variable Alternatives

In ES5, class variables were commonly defined using the FrameWork.Class() pattern. However, in ES6, class declarations do not natively support class variables.

Why the Omission?

The absence of class variables in ES6 was intentional. TC39 aimed to separate class declaration from member definition. Class definitions should define the class's capabilities, while members (both variables and methods) should be defined elsewhere.

Constructor Alternative

One option for defining class variables is to assign them within the constructor:

constructor() {
    this.MY_VAR = true;
}

However, this can clutter the constructor, especially with many class parameters.

ES7 Proposal

A proposed ES7 feature, "Property Initializers," provides a more concise syntax for defining instance variables in class declarations and expressions:

class MyClass {
    foo = bar;
}

Class Config Handler

An alternative approach is to create a class config handler that attaches variables to the class during initialization. For example, using a WeakMap:

const classConfig = new WeakMap();
class ConfiguredClass {
    constructor() {
        const config = classConfig.get(this.constructor);
        if (config) {
            for (const [key, value] of Object.entries(config)) {
                this[key] = value;
            }
        }
    }
}

class MyConfiguredClass extends ConfiguredClass {}
classConfig.set(MyConfiguredClass, {
    variable: 'string',
    variable2: true
});

Babel and TypeScript Support

Babel and TypeScript both provide syntax for defining class variables:

// Babel
class MyClass {
    varName = value;
}

// TypeScript
class MyClass {
    varName: string = value;
}

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