Ajax .post not returning data like array for JavaScript

I need a JavaScript variable like:

var test_js = {
    "first_test": "a_simple_test",
    "second_test": "other_simple_test",
    "last_test": "last_simple_test"
};

But the array is returned using ajax, i'm trying:

test.php

echo "
    'first_test': 'a_simple_test',
    'second_test': 'other_simple_test',
    'last_test': 'last_simple_test'
";

JavaScript

$.post("test.php").done(function(test_data) {
    var test_js = {
        test_data
    };
});

But when i'll minify the JavaScript (jscompress.com)

The following error is returned:

Unexpected token punc «}», expected punc «:»

How to do to fix it and return the array correctly?