[рони]

Spily,

function wave(str) {
    let symbol, waveArray = [], length = str.length;
    waveArray.push(str);
    str = str.toLowerCase();
    for (let i = 1; i < length; ) {
        symbol = str[i].toUpperCase();
        waveArray.push(str.slice(0, i) + symbol + str.slice(++i));
    }
    return waveArray;
}
console.log(wave('Hello'));
console.log(wave('Gap'));