js-102/030_functional_flow_control/030_003_catch.js

JS-102 helps you learn JavaScript (the right way) so that you can confidently use higher-level libraries and frameworks. — Let’s reveal the magic!

/*
 *                    the devil is in the details
 *      .--. __--__  (`-')    .--.  .----.  .----.
 *      | ,|/    _ / ( OO).->/_  | /  ..  \\_,-.  |
 *      |(_|\_..`--.(,------. |  ||  /  \  .  .' .'
 * ,--. |  |.-._)   \`------' |  |'  \  /  '.'  /_
 * |  '-'  /\       /         |  | \  `'  /|      |
 *  `-----'  `-----'          `--'  `---'' `------'
 *
 * This project is a part of the “Byte-Sized JavaScript” videocasts.
 *
 * You can watch “Byte-Sized JavaScript” at: https://bytesized.tv/
 *
 * MIT Licensed — See LICENSE.md
 *
 * Send your comments, suggestions, and feedback to me@volkan.io
 */

const util = require( '../lib/util' );
const separator = util.separator;
const log = console.log;

{
    separator();

    const underling = new Promise( ( resolve, reject ) => reject( 42 ) )

    const overlord = underling.catch( ( reason ) => {

        // This will result in `overlord` resolving with "hello".
        //
        // If you want to let this rejection cascade to `overlord`,
        // that’s not how you do it (see the second example below for that).
        return 'hello';
    } );

    overlord
        .then( ( data ) => {

            // Returning from the `cath` of the promise will let
            // the owner promise `resolve` with the returned value:
            log( `Resolved with “${data}”.` );

            separator();
        } )
        .catch( ( reason ) => {
            log( `Rejected with “${reason}”.` );

            separator();
        } );
}


{
    const underling = new Promise( ( resolve, reject ) => reject( 42 ) )

    const overlord = underling.catch(

        ( reason ) =>

        // For the owner promise to `reject`, return a rejected promise:
        Promise.reject( { reason, message: 'hello' } )

    );

    overlord
        .then( ( data ) => {
            log( `Resolved with “${data}”.` );
            separator();
        } )
        .catch( ( reason ) => {
            log( `Rejected with “${JSON.stringify(reason)}”.` );
            separator();
        } );
}