Reading/Web/Working with legacy code

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This page documents our experiences with working with modernizing legacy code.

jQuery specific migration[edit]

Migrating 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