The answer
A truthy or falsy value. If truthy, the element is included in the result. If falsy, it is excluded. If you return undefined (no return statement), nothing is included.
filter in JavaScript: Examples and Use Cases
What should the filter callback return in JavaScript?
More FAQs in filter in JavaScript: Examples and Use Cases
It calls a callback on each element and returns a new array with only the elements where the callback returned truthy. The original array is not mutated.
Use filter(Boolean). Boolean is a function that returns the truthiness of a value. Passing it to filter removes all falsy values (0, '', null, undefined, false, NaN).
filter returns all matching elements as an array (possibly empty). find returns the first matching element (or undefined). Use filter when you want all matches; use find when you want one.
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