Javascript return in eval string

Question

I know eval is evil, but in this case please do not point this out please. I am creating something very custom and in my case eval is the only solution (that is what eval is there for).

My problem is that return does not work in eval strings. The browser throws and Illegal return statement error. Could someone explain to me why this happens?

I would like this string to be eval'ed:

eval(`
  console.log('log this');
  return;
  console.log('DONT log this')
`);

Here is a stack blitz to play with

https://stackblitz.com/edit/angular-8x8p6m?file=src%2Fapp%2Fapp.component.ts

Return statements are only valid inside functions.

str
– str

2018-11-16 12:40:21 +00:00
Commented Nov 16, 2018 at 12:40 — str
– str, Commented Nov 16, 2018 at 12:40

puddi · Accepted Answer · 2018-11-16 12:42:22Z

3

Try inputting console.log('log this'); return; console.log('DONT log this'); in your browser (that's basically what eval is); it'll complain because you're trying to use return outside of a function.

However, you can do something like this:

eval(`
  (() => {
    console.log('log this');
    return;
    console.log('DONT log this');
  })()
`);

answered Nov 16, 2018 at 12:42

puddi

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1 Comment

Freddy Bonda Over a year ago

Most simplified answer. Thank you.

Shilly · Accepted Answer · 2018-11-16 12:41:51Z

2

This is why we use new Function() instead of eval() if we really need eval() for something.

If you pass a string to the Function constructor, it will automatically call eval() on it. So no need to wrap eval And function in the same statement, that's basically doing the same thing twice.

const js_str = `
  console.log('log this');
  return 'ok';
  console.log('DONT log this')
`;

const result = new Function( js_str )();

console.log( result );

If you could explain what you're creating with eval(), there might even be a better solution as well.

answered Nov 16, 2018 at 12:41

Shilly

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1 Comment

Freddy Bonda Over a year ago

Eval is faster to use is this case don't you think?

Jared Smith · Accepted Answer · 2018-11-16 12:41:59Z

1

Your problem is that you're not in a function:

eval('(new Function("console.log(1);return;console.log(2)"))()');

Logs 1 but not 2.

answered Nov 16, 2018 at 12:41

Jared Smith

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Comments

SiddAjmera · Accepted Answer · 2018-11-16 13:53:34Z

1

As mentioned in the comments, return statements are only valid inside functions

Give this a try:

eval('(function (){ 
  console.log("log this");
  return 23;
  console.log("do not log this"); 
})()');

edited Nov 16, 2018 at 13:53

SiddAjmera

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answered Nov 16, 2018 at 12:44

Arkady Spiridonov

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