javascript - 即使使用if语句也获得未定义的值

我将值推入forEach循环内的数组中。当我在循环外调用数组时，得到以下值：

d.data：

[{hello: 'abc', asd: '123', fgh: '345' }]
[{sdfg: '123', yo: 'ghj', fgh: '345' }]
[{gd: '123', asd: '123', bonjour: 'yui' }]
[{hello: '123', asd: '567', fgh: '345' }]

forEach循环：

let str_arr = [];
d.data.forEach(recs => {
  each_keys = Object.keys(recs);
  each_vals = Object.values(recs);
  each_vals.forEach(k => {
    if (typeof k == 'string') {
        find_key = Object.keys(recs).find(key => recs[key] === k);
        str_arr.push(find_key);
    }
   });
});

str_arr：

["hello", "hello", "hello"]
["yo", "yo", "yo"]
["bonjour", "bonjour", "bonjour"]
[]

console.log(typeof str_arr[0] + ' = ' + str_arr[0]

结果：

string = hello
string = yo
string = bonjour
undefined = undefined

我想将所有str_arr [0]推入一个数组中，所以我这样做吗？

let string_array = [];
if (str_arr[0] !== undefined) {
    string_array.push(str_arr[0]);
}
console.log(string_array);

我的结果是：

hello
yo
string
     (empty)

为什么我仍然在结果中得到未定义的值？

最佳答案

input_arr = [
  ["hello", "hello", "hello"],
  ["yo", "yo", "yo"],
  ["bonjour", "bonjour", "bonjour"],
  []
];
let output_array = [];
input_arr.forEach(element => {
  if (element[0] !== undefined) {
    output_array.push(element[0]);
  }
});
console.log(output_array);
console.log(input_arr[3][0] !== undefined); // works as expected