How to get a functions's body as string?

Question

I want to know how to convert a function\'s body into a string?

function A(){
  alert(1);
}

output = eval(A).toString() // this will come with  function A()         


        

Answer 1

Don't use a regexp.

const getBody = (string) => string.substring(
  string.indexOf("{") + 1,
  string.lastIndexOf("}")
)

const f = () => { return 'yo' }
const g = function (some, params) { return 'hi' }
const h = () => "boom"

console.log(getBody(f.toString()))
console.log(getBody(g.toString()))
console.log(getBody(h.toString())) // fail !

Answer 2

Currently, developers are using arrow functions with the new releases of Ecmascript.

Hence, I would like to share the answer here which is the answer of Frank

    function getArrowFunctionBody(f) {
      const matches = f.toString().match(/^(?:\s*\(?(?:\s*\w*\s*,?\s*)*\)?\s*?=>\s*){?([\s\S]*)}?$/);
      if (!matches) {
        return null;
      }
      
      const firstPass = matches[1];
      
      // Needed because the RegExp doesn't handle the last '}'.
      const secondPass =
        (firstPass.match(/{/g) || []).length === (firstPass.match(/}/g) || []).length - 1 ?
          firstPass.slice(0, firstPass.lastIndexOf('}')) :
          firstPass
      
      return secondPass;
    }
    
    const K = (x) => (y) => x;
    const I = (x) => (x);
    const V = (x) => (y) => (z) => z(x)(y);
    const f = (a, b) => {
      const c = a + b;
      return c;
    };
    const empty = () => { return undefined; };
    console.log(getArrowFunctionBody(K));
    console.log(getArrowFunctionBody(I));
    console.log(getArrowFunctionBody(V));
    console.log(getArrowFunctionBody(f));
    console.log(getArrowFunctionBody(empty));

Original question here

Answer 3

You could just stringify the function and extract the body by removing everything else:

A.toString().replace(/^function\s*\S+\s*\([^)]*\)\s*\{|\}$/g, "");

However, there is no good reason to do that and toString actually doesn't work in all environments.

Answer 4

If you're going to do something ugly, do it with regex:

A.toString().match(/function[^{]+\{([\s\S]*)\}$/)[1];