# 13.6.2 함수에 함수 전달

함수에 함수를 전달하는 함수 : 콜백 (callback) 

<예제>

```javascript
function sum (arr, f) {
    // 함수가 전달되지 않았으면 매개변수를 그대로 반환하는 null 함수를 사용
    if (typeof f != 'function') f = x => f
    
    return arr.reduce((a, x) => a += f(x), 0);
}

sum([1, 2, 3]); // 6
sum([1, 2, 3], x => x*x); // 14
sum([1, 2, 3], x => Math.pow(x, 3)); // 36
```


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