Transform array into object with true values

Let's assume that you have the following array:

javascript
let tags = ['React', 'JavaScript', 'Firebase'];

To transform the array into an object assigning true values:

javascript
// The comma operator evaluates from left to right:
// 1. Assigns 'true' to the keys
// 2. Returs the accumulator
let obj = tags.reduce((acc, tag) => (acc[tag] = true, acc), {});

console.log(obj);
// {React: true, JavaScript: true, Firebase: true}

The comma operator is useful to evaluate functions from left to right, returning the value of the last function.