structure.js

// I seriously doubt this is your actual data structure.

const data = [
  [name = 'john doe', name2 = 'hello'],
  [age = 23],
  [hobby = [{
    name: 'mancing'
  }]]
];
data.map((item, key) => console.log(item, key))

// The embedded names in those arrays are not "keys".  That's just a cheap
// way of creating new variables.  So, what you have written is exactly the 
// same as this:

name = 'john doe';
name2 = 'hello';
age = 23;
hobby = [{name: 'mancing'}];

const data = [
  [name, name2],
  [age],
  [hobby]
];

// And that seems wrong.