This page documents our experiences with working with modernizing legacy code.
jQuery specific migration
editMigrating code for ES6 compatible Promises
edit- For consistency, prefer to use
.then( successFn, failFn )when replacing.done( successFn ).fail( failFn )or.done( successFn ).catch( failFn ). Be aware of the return value in your callbacks though! - In general,
.done( function () { ... } ).fail( function () { ... } ).always( alwaysFn )can be replaced with.then( function () { ... alwaysFn() }, function () { ... alwaysFn() } ) - Be aware of Deferred handlers with multiple parameters. A Deferred can resolve with multiple arguments passed to its callback functions while a ES6 promise can only resolve with only one argument.
- When changing the return value of a function, keep in mind that
$.Deferred().done()returns a JQuery.Deferred, but$.Deferred().then()returns a JQuery.Promise so you may have to update the JSDocs. - You may need to convert tests from sync to async if the tests cover code that previously relied on .done/.fail. This is because .done/.fail callbacks may be called synchronously if the Deferred has already been resolved (which is often the case with test stubs), but .then/.catch callbacks are always called asynchronously. See jQuery upgrade guide for more info and commit d42523e609 for an example
- ESLint can be leveraged to forbid future usages of done, fail and always