What is the most elegant way to insert objects between array elements?

One-liner using plain ES6:

const interleave = (arr, thing) => [].concat(...arr.map(n => [n, thing])).slice(0, -1)

Usage:

interleave(['foo', 'bar', 'baz'], 'avocado')

Prints:

> ["foo", "avocado", "bar", "avocado", "baz"]