재귀함수 치환 모형

function plus(a, b) {
    return a === 0 ? b : plus(dec(a), inc(b));
}

function inc(v) {
    return v + 1;
}

function dec(v) {
    return v - 1;
}

/** return a === 0 ? b : plus(dec(a), inc(b));
 * plus(4, 5)
 * plus(dec(4), inc(5))
 * plus(3, 6)
 * plus(dec(3), inc(6))
 * plus(2, 7)
 * plus(dec(2), inc(7))
 * plus(1, 8)
 * plus(dec(1), inc(8))
 * plus(0, 9)
 * 9
 */



/** return a === 0 ? b : inc(plus(dec(a), b));
 * plus(4, 5) 
 * inc(plus(dec(4), 5))
 * inc(plus(3, 5))
 * inc(inc(plus(dec(3), 5)))
 * inc(inc(plus(2, 5)))
 * inc(inc(inc(plus(dec(2), 5))))
 * inc(inc(inc(plus(1, 5))))
 * inc(inc(inc(inc(plus(dec(1), 5)))))
 * inc(inc(inc(inc(plus(0, 5)))))
 * inc(inc(inc(inc(5))))
 * inc(inc(inc(6)))
 * inc(inc(7))
 * inc(8)
 * 9 
 * 
 * 
 */

해당 plus 함수는 재귀함수로 두 수의 합을 구하고 있다.