为什么我的find方法返回undefined？

Question

I'm recreating a number of Underscore.js methods to study JavaScript and programming in general.

Below is my attempts to recreate Underscore's _.find() method.

var find = function(list, predicate) { // Functional style
    _.each(list, function(elem){
        if (predicate(elem)) {
            return elem;
        }
    });
};

var find = function(list, predicate) { // Explicit style
    if (Array.isArray(list)) {
        for (var i = 0; i < list.length; i++) {
            if (predicate(list[i])) {
                return list[i];
            }
        }
    } else {
        for (var key in list) {
            if (predicate(list[key])) {
                return list[key];
            }
        }
    }
};

My second find method, which is using for loop and for in loop works. Whereas, my first find method would return undefined . I believe both should do the same work. However, they don't. Would someone please point what is going on?

Answer 1

Your return is only returning from the inner (nested) function and your find function is indeed not returning anything, hence the undefined .

Try this instead:

var find = function(list, predicate) { // Functional style
    var ret;

    _.each(list, function(elem){
        if (!ret && predicate(elem)) {
            return ret = elem;
        }
    });

    return ret;
};