Why can't you stringify a function expression?

Question

Why doesn't this produce anything?

console.log(JSON.stringify(function(){console.log('foobar');}));

Answer 1

JSON can't stringify functions at all, it handles them just like undefined or null values. You can check the exact algorithm at EcmaScript 5.1 §15.12.3, see also the description at MDN.

However you of course can stringify function expression by casting them to a string, try

console.log("" + function(){console.log('foobar');})

Answer 2

JSON has no means to represent a function. It is a data format designed for simplicity and compatibility across languages (and a function is the last thing that will be cross-language compatible).

From the docs for JSON.stringify:

If undefined, a function, or an XML value is encountered during conversion it is either omitted (when it is found in an object) or censored to null (when it is found in an array).

Answer 3

You cannot do that, but there are some third party libraries can help you do that, like: https://www.npmjs.com/package/json-fn

Answer 4

If you want to use JSON.stringify to also convert functions and native objects you can pass a converter function as the second argument:

const data = {
  fn: function(){}
}

function converter(key, val) {
  if (val && typeof val === 'function' || val.constructor === RegExp) {
    return String(val)
  }
  return val
}

console.log(JSON.stringify(data, converter, 2))

Return undefined from the converter function if you want to omit the result.

The third parameter is how many spaces you want the output to indent (optional).

Answer 5

yourFunctionName.toString(); will also stringify a function