prefer_contains

Always 'false' because 'indexOf' is always greater than or equal to -1.

Always 'true' because 'indexOf' is always greater than or equal to -1.

Unnecessary use of 'indexOf' to test for containment.

The analyzer produces this diagnostic when the method indexOf is used and the result is only compared with -1 or 0 in a way where the semantics are equivalent to using contains.

The following code produces this diagnostic because the condition in the if statement is checking to see whether the list contains the string:

void f(List<String> l, String s) {
  if (l.indexOf(s) < 0) {
    // ...
  }
}

Use contains instead, negating the condition when necessary:

void f(List<String> l, String s) {
  if (l.contains(s)) {
    // ...
  }
}

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.