Getting Undefined values even with if statement

I pushed values into an array inside of a forEach loop. When I call the array outside of the loop, I get these values:

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 Loop:

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]

Result:

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

I want to push all the str_arr[0] inside one array so I do this?

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

My result is:

hello
yo
string
     (empty)

Why am I still getting the undefined value in my results?