/**
 * Created by lizq on 2017/8/7
 */

// Callback index.
let count = 0;

/**
 * JSONP handler
 *
 * Options:
 * - prefix {String} callback prefix (defaults to `__jp`)
 * - callback {String} (defaults to `callback`)
 * - timeout {Number} how long after the request until a timeout error
 *   is emitted (defaults to `15000`)
 */
function jsonp( url, options ) {
    options      = options || {};
    let prefix   = options.prefix || '__jp';
    let callback = options.callback || 'callback';
    let params   = options.data || {};
    let timeout  = options.timeout ? options.timeout : 15000;
    let target   = document.getElementsByTagName( 'script' )[ 0 ] || document.head;
    let script;
    let timer;
    let promise;
    // Generate a unique id for the request.
    let id       = prefix + (count++);

    function noop() {}

    function cleanup() {
        // Remove the script tag.
        if ( script && script.parentNode ) {
            script.parentNode.removeChild( script );
        }
        window[ id ] = noop;
        if ( timer ) { clearTimeout( timer ); }
    }

    function serialize( params ) {
        let param = '';
        for ( let key in params ) {
            if ( params.hasOwnProperty( key ) ) {
                param += `&${key}=${encodeURIComponent( params[ key ] )}`;
            }
        }
        return param;
    }

    promise = new Promise( ( resolve, reject ) => {
        if ( timeout ) {
            timer = setTimeout( () => {
                cleanup();
                reject( new Error( 'Timeout' ) );
            }, timeout );
        }
        window[ id ]       = function( data ) {
            cleanup();
            resolve( data );
        };
        params[ callback ] = id;
        url += serialize( params );
        url                = url.replace( '?&', '?' );
        // Create script.
        script             = document.createElement( 'script' );
        script.src         = url;
        script.onerror     = function() {
            cleanup();
            reject( new Error( 'Network Error' ) );
        };
        target.parentNode.insertBefore( script, target );
    } );
    return promise;
}

export default jsonp;