How can I use js eval to return a value?

Question

I need to evaluate a custom function passed from the server as a string. It's all part of a complicated json I get, but anyway, I seem to be needing something along the lines:

var customJSfromServer = "return 2+2+2;"
var evalValue = eval(customJSfromServer);
alert(evalValue) ;// should be "6";

Obviously this is not working as I expected. Any way I can achieve this ?

score 91 · Accepted Answer · answered Sep 13 '11 at 08:46

91

The first method is to delete return keywords and the semicolon:

var expression = '2+2+2';
var result = eval('(' + expression + ')')
alert(result);

note the '(' and ')' is a must.

or you can make it a function:

var expression = 'return 2+2+2;'
var result = eval('(function() {' + expression + '}())');
alert(result);

even simpler, do not use eval:

var expression = 'return 2+2+2;';
var result = new Function(expression)();
alert(result);

answered Sep 13 '11 at 08:46

otakustay

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2

Thanks for this. In your second example, the `eval('(function() {' + expression + '}())');` fails in case there is a comment on the last line of the expression, with an error like **unexpected end of input**. To fix it, add a newline to the end: `'\n}())'`. – gregn3 Dec 13 '16 at 19:05
Thanks for the solution ,i used second example ,with a new line '\n}())' at the end and it worked fine. – Muhammad Mufeez Ahmed Jan 02 '18 at 16:09

score 10 · Answer 2 · answered Sep 13 '11 at 08:45

If you can guarantee the return statement will always exist, you might find the following more appropriate:

var customJSfromServer = "return 2+2+2;"
var asFunc = new Function(customJSfromServer);
alert(asFunc()) ;// should be "6";

Of course, you could also do:

var customJSfromServer = "return 2+2+2;"
var evalValue = (new Function(customJSfromServer)());
alert(evalValue) ;// should be "6";

score 3 · Answer 3 · answered Sep 13 '11 at 08:44

3

var customJSfromServer = "2+2+2;"
var evalValue = eval(customJSfromServer);
alert(evalValue) ;// should be "6";

answered Sep 13 '11 at 08:44

chim

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3
49
59

score 2 · Answer 4 · answered Sep 13 '11 at 08:43

2

There should not be return statement , as eval will read this as statment and will not return value.

var customJSfromServer = "2+2+2;"
var evalValue = eval( customJSfromServer );
alert(evalValue) ;// should be "6";

see http://www.w3schools.com/jsref/jsref_eval.asp

answered Sep 13 '11 at 08:43

SergeS

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35

That probably also works if you declare `evalValue` outside of the string. – Thilo Sep 13 '11 at 08:45

score 2 · Answer 5 · answered Sep 13 '11 at 08:44

2

This works:

function answer() {
    return 42;
}

var a = eval('answer()');
console.log(a);

You need to wrap the return inside a function and it should pass the value on from the eval.

answered Sep 13 '11 at 08:44

Alex Ciminian

11,087
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93

score 1 · Answer 6 · edited Sep 13 '11 at 10:52

1

Modify server response to get "2+2+2" (remove "return") and try this:

var myvar = eval(response);
alert(myvar);

edited Sep 13 '11 at 10:52

genesis

49,367
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answered Sep 13 '11 at 08:47

Matjaz Muhic

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score -1 · Answer 7 · answered Jun 27 '19 at 11:01

-1

In 2019 using Template Literals:

var customJSfromServer = "2+2+2;"
var evalValue = eval(`${customJSfromServer}`);
alert(evalValue) ;// should be "6";

answered Jun 27 '19 at 11:01

leoncc

227
3
6

How can I use js eval to return a value?

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