Get function from string in JavaScript

Question

I have some JavaScript function definitions stored in a string.

"function firstFun() {
  return 'some text';
}

function secondFun() {
  console.log(firstFun())
}"

Is it possible to access and call these functions like any other function? Like:

var text = firstFun(); // return 'some text'
secondFun();           // print 'some text'

You could (but should not) use eval or parse the string by yourself. However, I would strongly recommend to reconsider your design. — str
– str, Commented Aug 6, 2018 at 11:19
It's possible, but should be avoided in almost all situations. Which problem should 'creating functions from strings' solve? — Shilly
– Shilly, Commented Aug 6, 2018 at 11:19

hsz · Accepted Answer · 2018-08-06 11:20:03Z

4

If you have stored JS code in string, you'll have to eval it (which is a dangerous operation - if you're running untrusted code as T.J. Crowder clarified):

const code = `
  function firstFun() {
    return 'some text';
  }

  function secondFun() {
    console.log(firstFun())
  }
`;

eval(code);

edited Aug 6, 2018 at 11:20

answered Aug 6, 2018 at 11:19

hsz

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4 Comments

T.J. Crowder Over a year ago

"which is a dangerous operation" Only if the content you're evaling is from an untrusted source.

hashtagslam Over a year ago

After I evaled how should I call the function? I tried the default way like I stated in my question, but I get an exception that "firstFun is not defined".

hsz Over a year ago

firstFun should be available in the context.

hashtagslam Over a year ago

If anyone got the exception I had, maybe you're using strict mode and that's why it fails.

eisbehr · Accepted Answer · 2018-08-06 11:30:37Z

Various ways ... even depending if you can change the format of the strings. But this should give you an idea. Keep in mind, that this can be dangerous, if you add such functionality to your scripts.

With eval:

const code = `
  function firstFun() {
    return 'some text';
  }

  function secondFun() {
    console.log(firstFun())
  }
`;

eval(code);
secondFun();

Putting it in a script tag:

const code = `
  function firstFun() {
    return 'some text';
  }

  function secondFun() {
    console.log(firstFun())
  }
`;

var head = document.querySelector('head');
var script = document.createElement('script');
script.type = 'text/javascript';
script.innerHTML = code;
head.appendChild(script);

secondFun();

Use Function constructor:

const firstStr = `return 'some text';`;
const secondStr = `console.log(firstFun())`;

var firstFun = new Function(firstStr);
var secondFun = new Function(secondStr);

secondFun();

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