Javascript代碼需要的解釋（遞歸示例）

Question

我的主要問題是，我似乎無法理解如何解決紙上的問題，更別說理解代碼，或寫我自己的。下面是我正在閱讀的一本書Eloquent JavaScript的摘錄。

考慮一下這個難題：從數字1開始，反覆添加5或乘以3，可以產生無限量的新數字。你如何編寫一個函數，給定一個數字，試圖找出產生該數字的一系列加法和乘法？

¶例如，可以通過首先將1乘以3，然後再加5兩次來達到數字13。數字15根本無法達到。

¶下面是解：

function findSequence(goal) { 
    function find(start, history) { 
    if (start == goal) 
     return history; 
    else if (start > goal) 
     return null; 
    else 
     return find(start + 5, "(" + history + " + 5)") || 
       find(start * 3, "(" + history + " * 3)"); 
    } 
    return find(1, "1"); 
} 

print(findSequence(24)); 

Answer 1

function findSequence(goal) { 

    // define a function that has a 'start' number (current total), 
    // and a string that is the history of what we've done so far 
    // (and note that for this function, the 'goal' parameter above is in scope). 
    function find(start, history) { 
    // if we've reached the goal, return the string that says how we got there 
    if (start == goal) 
     return history; 
    // if we've overshot, return null 
    else if (start > goal) 
     return null; 
    else 
     // call this same function (recursion) with two different possibile ways of 
     // getting closer to the goal - one adding 5 and one multiplying by 3... 
     // the or ('||') operator will return the first of the two that is not null 
     return find(start + 5, "(" + history + " + 5)") || 
       find(start * 3, "(" + history + " * 3)"); 
    } 
    // start at 1 
    return find(1, "1"); 
}