uneval

Convert value into evaluable string.

Table of contents

• Presentation
• JSON.stringify limits
• Browser example
• Node example
• Installation

Presentation

@jsenv/uneval turns a JavaScript value into a string that can be evaluated. It exists to overcome JSON.stringify limitations.

Using @jsenv/uneval + eval is more powerful and accurate than JSON.stringify + JSON.parse. It supports circular structure and preserves types like Date, Infinity, -0, BigInt and more.

However JSON.stringify is way faster and is safe (it cannot execute arbitrary code). So prefer JSON.stringify + JSON.parse over uneval + eval when you can.

JSON.stringify limits

• Returns "{}" for a regexp

  JSON.stringify(/foo/) === "{}"

• Returns "0" for -0

  JSON.stringify(-0) === "0"

• Returns "null" for NaN

  JSON.stringify(NaN) === "null"

• Returns "null" for Infinity

  JSON.stringify(Infinity) === "null"

• Returns a string for a date

  JSON.stringify(new Date(0)) === `"1970-01-01T00:00:00.000Z"`

• Throws on BigInt

  try {
    JSON.stringify(1n)
  } catch (e) {
    e.type // "TypeError"
    e.message // "Do not know how to serialize a BigInt"
  }

• Throws on circular structure

  const value = {}
  value.self = value
  try {
    JSON.stringify(value)
  } catch (error) {
    error.name // "TypeError"
    error.message // "Converting circular structure to JSON"
  }

• Ignores non enumerable properties

  JSON.stringify(Object.defineProperty({}, "foo", { enumerable: false })) === "{}"

• Is not optimized for repetitive structure

  const value = "a-very-long-string"
  JSON.stringify([value, value])

  "a-very-long-string" would be repeated twice the string.

Browser example

<script src="https://unpkg.com/@jsenv/[email protected]/dist/global/main.js"></script>
<script>
  const { uneval } = window.__jsenv_uneval__
  console.log(eval(uneval({ answer: 42 })))
</script>

— see also https://jsenv.github.io/jsenv-uneval/browser-example.

Node example

const { uneval } = require("@jsenv/uneval")

console.log(eval(uneval({ answer: 42 })))

— see also https://jsenv.github.io/jsenv-uneval/node-example.

Installation

npm install @jsenv/uneval