Promise in ES 6

promise-in-es-6.markdown

Promise in ES 6

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License.

script.js

(() => {

    class Promise {

        constructor(main) {

            this.result = null;
            this.state = -1;
            this.callback = [ ];

            const end = (type, result) => {

                if (this.state > -1)  return;

                this.state = type, this.result = result;

                setTimeout(() => {

                    for (let func of this.callback)  func[type]( result );

                    this.callback.length = 0;
                });
            };

            main(end.bind(null, 0),  end.bind(null, 1));
        }

        then(resolve, reject) {

            return  new Promise((_resolve_, _reject_) => {

                const callback = [
                    value => _resolve_(resolve( value )),
                    error => _reject_(reject( error ))
                ];

                if (this.state < 0)
                    this.callback.push( callback );
                else
                    callback[this.state]( this.result );
            });
        }
    }


    const promise = new Promise((resolve, reject) => {

        setTimeout(() => resolve(1));

        setTimeout(() => reject(0));
    });

    function print(...text) {
        
        console.log(...text);
        
        document.body.append(text.join(' '), document.createElement('br'));
    }
    
    promise.then(
        value => print(value) || 2,
        error => print(error)
    ).then(
        value => print(value)
    );
    
    setTimeout(
        () => promise.then(value => print(value)).then(() => print(2))
    );
})();