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Change async pipe's default value from null to undefined #39785
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josef-kriz
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You are correct that Could you get the tests green? Once the tests are green we can try to see what it would take to land it in Google codebase, but I suspect it will not be trivial. |
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It is an incorrect approach to return `null` when the actual value is unknown. The convention is to use `undefined` for a variable that has not yet been assigned a value, whereas `null` is an assignment value and represents no value. BREAKING CHANGE: The async pipe now returns `undefined` as the default value. Requires a fix when relying on the async pipe returning `null` as the default value. Fixes #16982
mhevery
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Changes async pipe's default value from
nulltoundefined. It is an incorrect approach to returnnullwhen the actual value is unknown. The convention is to useundefinedfor a variable that has not yet been assigned a value, whereasnullis an assignment value and represents no value (more details for example on Stack Overflow).PR Checklist
Please check if your PR fulfills the following requirements:
PR Type
What kind of change does this PR introduce?
What is the current behavior?
Angular's async pipe subscribes to Observables and Promises and returns the latest value. But when there is no value yet, it returns
null. This can lead to some unexpected issues, for example imagine a component with an input like this:The component can then be used in a template without any problems like this (where myNumber is type of
number):But when myNumber would be of type
Observable<number>and we'd like to use it with the async pipe, it would lead tonullbeing passed to the number component instead ofundefined:A typical use case for this is firing an async HTTP request, displaying some loading element for value of
undefined, a value for the actual value, and anything else ("no data", error, ...) fornull. The current async pipe breaks this pattern and can cause a headache finding what went wrong as this is not the expected behavior.Issue Number: #16982
What is the new behavior?
The async pipe now returns
undefinedby default before an actual value is emitted. It still returnsnullfor inputs of typenullandundefinedto keep it consistent with other pipes (PR #37447).This PR follows @mhevery's comment to see how breaking it is. As discussed in the issue #16982, it is not fully clear whether this should be a fix or a feature.
Does this PR introduce a breaking change?
The async pipe now returns
undefinedas the default value. Requires a fix if you performed a check for thenullvalue before, but this check was likely needed solely because of this unconventional behavior.Other information
I've also updated the doc comment that this pipe returns
undefinedbefore the first emit, as I was lacking this information aboutnullthere before.