diamond.js

function renderDiamond(input) {

    let preSpace = (input - 1) / 2
    let postSpace = (input - 1) / 2
    let output = ""
    let f = (input - 1) / 2

    for (var line = 0; line < input; line++) {

        for (var col = 0; col < input; col++) {

            if (col < preSpace || col > postSpace) {
                output = output + "_"
            } else {
                output = output + "*"
            }
        }

        output = output + "\n"
        if (line < f) {
            preSpace = preSpace - 1
            postSpace = postSpace + 1
        } else {
            preSpace = preSpace + 1
            postSpace = postSpace - 1
        }
    }
    console.log(output)
}