Help Jason format his JSON

Question

Jason has a big JSON but it's unreadable, so he needs to prettify it.

Formatting Spec

The JSON has 4 different types:

• Numbers; Just 0-9
• Strings; Double quoted " strings escaped with \
• Arrays; Delimited by [], with items separated by ,, items can be any of these types
• Objects; Delimited by {}, format is key: value where key is a string and value is any of these types

Spacing

• Arrays should have exactly one space after the commas between items
• Objects should have have just one space between the key and the value, after the :

Indentation

• Each nesting level is indented 2 more than the previous
• Each object key/value pair is always on its own line. Objects are indented
• An array is indented across multiple lines if it contains another array or object. Otherwise the array remains on one line

Rules

• Built-ins which trivialize this task are not allowed.
• As always standard loopholes are disallowed

Examples

[1,2,3]
[1, 2, 3]

{"a":1,"b":4}
{
  "a": 1,
  "b": 4
}

"foo"
"foo"

56
56

{"a":[{"b":1,"c":"foo"},{"d":[2,3,4,1], "a":["abc","def",{"d":{"f":[3,4]}}]}]}
{
  "a": [
    {
      "b": 1,
      "c": "foo"
    },
    {
      "d": [2, 3, 4, 1],
      "a": [
        "abc",
        "def",
        {
          "d": {
            "f": [3, 4]
          }
        }
      ]
    }
  ]
}

[2,["foo123 ' bar \" baz\\", [1,2,3]]]
[
  2,
  [
    "foo123 ' bar \" baz\\",
    [1, 2, 3]
  ]
]

[1,2,3,"4[4,5]"]
[1, 2, 3, "4[4,5]"]

[1,2,3,{"b":["{\"c\":[2,5,6]}",4,5]}]
[
  1,
  2,
  3,
  {
    "b": ["{\"c\":[2,5,6]}", 4, 5]
  }
]

Answer 1

JavaScript (ES6), 368 bytes

f=(s,r=[],i='',j=i+'  ',a=[])=>s<'['?([,,r[0]]=s.match(s<'0'?/("(?:\\.|[^"])*")(.*)/:/(\d+)(.*)/))[1]:s<'{'?(_=>{for(;s<']';s=r[0])a.push(f(s.slice(1),r,j));r[0]=s.slice(1)})()||/\n/.test(a)?`[
${j+a.join(`,
`+j)}
${i}]`:`[${a.join`, `}]`:(_=>{for(a=[];s<'}';s=r[0])a.push(f(s.slice(1),r,j)+': '+f(r[0].slice(1),r,j));r[0]=s.slice(1)})()||`{
${j+a.join(`,
`+j)}
${i}}`

Less golfed:

function j(s, r=[], i='') { // default to no indentation
    if (s < '0') { // string
        let a = s.match(/("(?:\\.|[^"])*")(.*)/);
        r[0] = a[2]; // pass the part after the string back to the caller
        return a[1];
    } else if (s < '[') { // number
        let a = s.match(/(\d+)(.*)/);
        r[0] = a[2]; // pass the part after the string back to the caller
        return a[1];
    } else if (s < '{') { // array
        let a = [];
        while (s < ']') { // until we see the end of the array
            s = s.slice(1);
            a.push(j(s, r, i + '  ')); // recurse with increased indentation
            s = r[0]; // retrieve the rest of the string
        }
        r[0] = s.slice(1); // pass the part after the string back to the caller
        if (/\n/.test(a.join())) { // array contained object
            return '[\n  ' + i + a.join(',\n  ' + i) + '\n' + i + ']';
        } else {
            return '[' + a.join(', ') + ']';
        }
    } else { // object
        let a = [];
        while (s < '}') { // until we see the end of the object
            s = s.slice(1);
            let n = j(s, r, i + '  ');
            s = r[0].slice(1);
            let v = j(s, r, i + '  ');
            a.push(n + ': ' + v);
            s = r[0]; // retrieve the rest of the string
        }
        r[0] = s.slice(1); // pass the part after the string back to the caller
        return '{\n  ' + i + a.join(',\n  ' + i) + '\n' + i + '}';
    }
}