[Nexus]

fluorke,

function func(string) {
    let position = -1;
    do {
        const before = string.substring(0, position + 1); //with space
        const needle = string.substr(position + 1, 1);
        const after = string.substring(position + 2);

        string = before + needle.toUpperCase() + after;
    } while ((position = string.indexOf(' ', position + 1)) >= 0);

    return string;
};

alert(
    func('вася петя маша таня')
);