is-generator-es6/index.js

Test if your value is a ES6:Generator or ES6:GeneratorFunction

/********************************************
 * Created by GUERIN Olivier, on 02/11/2015.*
 * Twitter: @MisterRaton                    *
 ********************************************/

var isBrowser = typeof window === "object",
    functionToString = Function.prototype.toString,
    objectToString = Object.prototype.toString,
    generatorFunctionConstructor,
    generatorFunctionPrototype;

try {

    //Initialize generatorFunctionConstructor
    //Never use eval -> decrease perf.
    var generatorFunction = new Function( "return (function* (){})" )();
    generatorFunctionConstructor = generatorFunction.constructor;

    if ( typeof generatorFunctionConstructor === "function" ) {

        //Initialize generatorFunctionPrototype
        if ( typeof generatorFunction.prototype === "object" && typeof generatorFunction.prototype.__proto__ === "object" ) {
            generatorFunctionPrototype = generatorFunction.prototype.__proto__;
            if ( generatorFunctionPrototype.hasOwnProperty( "next" ) === false || generatorFunctionPrototype.hasOwnProperty( "throw" ) === false ) {
                generatorFunctionPrototype = null;
            }
        }
    } else {
        generatorFunctionConstructor = noop;
        generatorFunctionPrototype = null;
    }

} catch ( err ) {
    generatorFunctionConstructor = noop;
    generatorFunctionPrototype = null;
}

function noop() {
}

/**
 * Test if the value is an instanceof ES6:GeneratorFunction or start with function*
 * @param {*} value
 * @returns {boolean} Returns if value is a GeneratorFunction
 */
function isGeneratorFunction( value ) {

    if ( value instanceof generatorFunctionConstructor ) {
        return true;

    } else if ( isBrowser === true ) {

        //Handle values from another frame: checks the presence of next & throw, then checks if the function start with "function*"
        if ( typeof value === "function" && typeof value.prototype === "object" && "next" in value.prototype && "throw" in value.prototype ) {
            return /^function\*/.test( functionToString.call( value ) );
        }
    }
    return false;
}

/**
 * Test if the value is a ES6:Generator after checking the presence of next and throw by callinsg object.prototype.toString
 * @param {*} value
 * @returns {boolean} Returns if value is a Generator
 */
function isGenerator( value ) {

    if ( typeof value === "object" && "throw" in value && "next" in value ) {
        return objectToString.call( value ) === "[object Generator]";
    }
    return false;
}
/**
 * Tests if the value is a ES6:Generator by comparing the prototype of the argument with the prototype of GeneratorFunction
 * If the test failed inside a navigator, the code will execute `Object.prototype.toString`
 * @param {*} value
 * @returns {boolean} Returns if value is a Generator
 */
function _isGenerator( value ) {

    //Shortcut if the prototype of function* couldn't be found
    if ( generatorFunctionPrototype === null ) {
        return isGenerator( value );
    }

    //Checks the presence of "throw" and "next" in the object
    if ( typeof value === "object" && "throw" in value && "next" in value ) {

        //Retrieve the second prototype in the chain and compare it
        if ( typeof value.__proto__ === "object" && value.__proto__.__proto__ === generatorFunctionPrototype ) {
            return true;
        }

        if ( isBrowser === true ) {
            return objectToString.call( value ) === "[object Generator]";
        }
    }
    return false;
}

module.exports = isGenerator;
module.exports._isGenerator = _isGenerator;
module.exports.isGeneratorFunction = isGeneratorFunction;