`when`

Applies a callback when a predicate holds, otherwise returns the value unchanged.

T when<T>(bool Function(T) predicate, T Function(T) callback, T value)

Lecture

when(predicate, callback, value) is a single conditional expressed as data: if predicate(value) is true, the result is callback(value); otherwise the result is value itself, untouched. It reads nicely inside a map callback when you want to transform only the elements that need it and leave the rest alone.

FxTS's version can widen the return type to a union T | R when the branches disagree. Dart has no union types, so both the callback branch and the pass-through branch must share the same type T — if you need genuinely different result types, reach for cases instead, which allows R to differ from the input type.

Demo 1 · Basics

Demo 2 · Clamping values in a pipeline

Try it yourself

Exercise: use when to double only the positive prices, leaving negative ones untouched.

Related: unless — the inverse-condition sibling of when · cases — multiple predicates, and a differing result type · not / negate — build the predicate you pass in · map — the usual place to use when as a callback