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babel-plugin-transform-decorators-legacy

Babel Legacy Decorator plugin

This is a plugin for Babel 6 that is meant to replicate the old decorator behavior from Babel 5 in order to allow people to more easily transition to Babel 6 without needing to be blocked on updates to the decorator proposal or for Babel to re-implement it.

Why "legacy"?

Decorators are still only a relatively new proposal, and they are (at least currently) still in flux. Many people have started to use them in their original form, where each decorator is essentially a function of the form

function(target, property, descriptor){}

This form is very likely to change moving forward, and Babel 6 did not wish to support the older form when it was known that it would change in the future. As such, I created this plugin to help people transition to Babel 6 without requiring them to drop their decorators or requiring them to wait for the new proposal update and then update all their code.

Best Effort

This plugin is a best effort to be compatible with Babel 5's transpiler output, but there are a few things that were difficult to reproduce, and a few things that were simply incorrect in Babel 5 with respect to the decorators proposal.

Two main things to mention as differences, though not things you are likely to encounter:

  1. Decorators expressions are evaluated top to bottom, and executed bottom to top. e.g.

    function dec(id){
        console.log('evaluated', id);
        return (target, property, descriptor) => console.log('executed', id);
    }
     
    class Example {
        @dec(1)
        @dec(2)
        method(){}
    }

    In Babel 5, this would output:

    evaluated 2
    evaluated 1
    executed 2
    executed 1

    With this plugin, it will result in:

    evaluated 1
    evaluated 2
    executed 2
    executed 1

    which is what the spec dictates as the correct behavior and was incorrect in Babel 5.

  2. Static class property initializers are evaluated once up front.

    If you decorate a static class property, you will get a descriptor with an initializer property. However whereas with Babel 5 this could be re-executed multiple times with potentially differing results, decorators-legacy will precompute the value and return an initializer that will return that value. e.g.

    function dec(target, prop, descriptor){
        let {initializer} = descriptor;
        delete descriptor.initializer;
        delete descriptor.writable;
     
        descriptor.get = function(){
            return initializer.call(this);
        };
    }
     
    var i = 0;
     
    class Example {
        @dec
        static prop = i++;
    }

    In Babel 5, every access to prop would increment i. In Babel 6, the very first value of i will be cached for future initializer calls.

    The spec is a little vague around how initializers work for repeat calls, and I'd consider calling an initializer multiple times to be a mistake in general, so hopefully this will not cause anyone trouble.

License

MIT (c) 2015