JSON Cheat Sheet

Practical Snippet

Key points: (1) JSON accepts only double quotes, no comments and no trailing commas — JavaScript habits that make strict parsers fail outright; (2) integers above 2^53 silently lose precision in most parsers, so large values such as snowflake IDs, order numbers and minor currency units should travel as strings; (3) passing locally does not mean passing in production, since permissive and strict parsers disagree; (4) validate inbound payloads before use and strip __proto__ keys against prototype pollution; (5) normalise outbound with JSON.stringify, which never emits trailing commas.

// ① 语法速查：合法 vs 常见错误
const ok = { s: "double only", n: 1e3, b: true, z: null, a: [1, 2] }
const bad = {
  // single: 'not valid JSON',      // ❌ 单引号
  // n1: 0800,                    // ❌ 非法数字（前导 0）
  // t: undefined,                // ❌ undefined 不存在
  // a: [1, 2, 3,],               // ❌ 尾随逗号
}
// console.log(JSON.stringify(bad))   // 注释被丢掉，undefined 被丢掉

// ② 🔴 大整数精度丢失（静默发生，不报错）
const big = '{"id":9007199254740993}'          // 2^53 + 1
const parsed = JSON.parse(big)
console.log(parsed.id)                         // 9007199254740992 ← 末位丢了
console.log(parsed.id === 9007199254740993n)   // false
// ✅ 方案：传字符串
console.log(JSON.parse('{"id":"9007199254740993"}').id)   // '9007199254740993'

// ③ 转义：控制字符与引号必须转义
const risky = { note: 'He said "hi"
and left' }
console.log(JSON.stringify(risky))
// {"note":"He said "hi"
and left"}   ← 正确转义

// ④ 解析前校验：别等运行时才发现格式错误
function parseJsonSafely(text) {
  try {
    const v = JSON.parse(text)
    if (v === null || typeof v !== 'object') {
      return { ok: false, error: '顶层必须是对象或数组' }
    }
    return { ok: true, value: v }
  } catch (e) {
    return { ok: false, error: String(e.message) }
  }
}
console.log(parseJsonSafely('{"a":1}'))        // { ok: true, ... }
console.log(parseJsonSafely('{"a":1,}'))       // { ok: false, ... }

// ⑤ 出站前规范化：stringify 不会产生尾随逗号
console.log(JSON.stringify({ a: 1, b: [1, 2] }, null, 2))

// ⑥ 深度解析与原型污染防护（来自不可信输入时）
const safe = JSON.parse(text, (k, v) => (k === '__proto__' ? undefined : v))