Scans from right, until a test passes. :package: :smiley_cat: :running: :vhs: :moon: :scroll: :newspaper: :blue_book:

Alternatives: scanWhile, scanWhileRight, scanUntil, scanUntilRight. Similar: search, scan, find.

This is part of package extra-array.

array.scanUntilRight(x, ft);
// x:  an array
// ft: test function (v, i, x)
// → first index where test fails till end

const array = require("extra-array");

var x = [1, 1, 2, 2, 3, 3, 4, 4];
array.scanUntilRight(x, v => v % 3 === 0);
// 6                       ^

array.scanUntilRight(x, v => v % 2 === 0);
// 8                            ^

array.scanUntilRight(x, v => v % 6 === 0);
// 0    ^

References

• List-Extra.elemIndex: elm
• iterable-binsearch: @krisselden
• binarysearch: @soldair