Diamond kata

diamond.js

// Simple use (from letter A, space as filler):
// diamond()("F")
// Specify beginning of alphabet and different filler character:
// diamond("F", "_")("K")

const diamond = (startChar="A", whiteSpace=" ") => {
    const range = (from, to) => [from]
        .concat(Array(to-from).fill(from))
        .map((n,index) => n+index)

    const codeToLetter = code => String.fromCharCode(code)

    const prepareLine = (letter, index, {length}) => {
        const line = whiteSpace.repeat(length * 2 - 1) // 2n - 1
        return [line, letter]
    }

    const insertLetter = ([line, letter], index, {length}) => {
        const lineArr = line.split('')
        const midIndex = length - 1
        lineArr.splice(midIndex - index, 1, letter)
        lineArr.splice(midIndex + index, 1, letter)
        return lineArr.join('')
    }
    
    return letter => {
        const top = range(startChar.charCodeAt(0), letter.charCodeAt(0))
            .map(codeToLetter)
            .map(prepareLine)
            .map(insertLetter)
            
        const bottom = top.slice(0,-1).reverse()
        return top.concat(bottom).join('\n')
    }
}

diamond2.js

const diamond = (startChar="A", whiteSpace=" ") => {
    const range = (from, to) => [from]
          .concat(Array(to-from).fill(from))
          .map((n,index) => n+index)

    const codeToLetter = code => String.fromCharCode(code)
    
    const mirrorH = str => str.split('').reverse().join('')

    const prepareLine = (letter, index, {length}) => {
        const line = whiteSpace.repeat(length).split('')
        line.splice(length - index - 1, 1, letter)
        return line.join('')
    }
        
    return letter => {
        const topLeftQuarter = range(startChar.charCodeAt(0), letter.charCodeAt(0))
            .map(codeToLetter)
            .map(prepareLine)
            
        const top = topLeftQuarter.map(s => s + mirrorH(s).substr(1))
        const bottom = top.slice(0, -1).reverse()
        return top.concat(bottom).join('\n')
    }
}