What does console.log([true, false].reduce((a, b) => a +… output?

Console output

Why

reduce starts at 0 and adds each element. Because + with a number and a boolean triggers numeric coercion, true counts as 1 and false counts as 0, giving 0 + 1 + 0 = 1. Summing booleans this way is a handy idiom for counting how many array items satisfy a condition.