Youre array declaration isn't correct. This is the correct syntax for declaring an array in JS and using JSON.stringify on it:

tab = [
  {city: 'Paris', zip: '75000'},
  {city: 'Toulouse', zip: '31000'},
  {city: 'Marseille', zip: '13000'}
];

JSON.stringify(tab, null, 2)

You could use Array.prototype.map to convert sub-array entries of the original array into objects with suitably named properties, and call JSON.stringify on the result.

const tabs = [
  [
    "Paris",
    "75000"
  ],
  [
    "Toulouse",
    "31000"
  ],
  [
    "Marseille",
    "13000"
  ]
];

// restructure sub-arrays into objects:

let objectArray = tabs.map(entry=> {
    const [city, zip] = entry;
    return {city, zip};
})

// Stringify object array

let jsonText = JSON.stringify( objectArray, null, 2)
console.log(jsonText);  

The map function is using Destructuring assignment to extract city and zip values from each sub-array.

A null second and numeric third parameter supplied to JSON.stringify improve human readability of the output but are generally omitted in production environments to reduce the length of network messages.