Character between every char (ignores pre-existing spaces)

// ok the code is now 100% more better

const specialchar = (str="Test String", char=" ") => {
    if (typeof str != "string" || typeof char != "string" || char.length > 1) return undefined
    else return str.split("").join(char)
}

// example
// code | expected output

specialchar()                   // T e s t   S t r i n g
                                //
specialchar("foo")              // f o o
specialchar("bar")              // b a r
specialchar("foobar")           // f o o b a r
                                //
specialchar("Test String", "_") // T_e_s_t_ _S_t_r_i_n_g
specialchar("foo", "_")         // f_o_o
specialchar("bar", "_")         // b_a_r
specialchar("foobar", "_")      // f_o_o_b_a_r