How to get function body text in JavaScript?

Question

function derp() { a(); b(); c(); }

derp.toString() will return "function derp() { a(); b(); c(); }", but I only need the body of the function, so "a(); b(); c();", because I can then evaluate the expression. Is it possible to do this in a cross-browser way?

@user1600680: `;` separates statements and hence cannot be part of an expression. Everything that throws a syntax error when put into the grouping operator `(...)` is not a valid expressions. But maybe the OP did not refer to this technical meaning with the term *expression*. — Felix Kling, Sep 01 '12 at 12:42
Oh yeah, I guess to be correct there should be an "s" at the end of "expression" since it's 3 separate expression statements. — gray state is coming, Sep 01 '12 at 12:48

score 55 · Accepted Answer · edited May 23 '17 at 11:46

55

var entire = derp.toString(); 
var body = entire.slice(entire.indexOf("{") + 1, entire.lastIndexOf("}"));

console.log(body); // "a(); b(); c();"

Please use the search, this is duplicate of this question

edited May 23 '17 at 11:46

Community

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answered Sep 01 '12 at 12:34

divide by zero

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Use `.slice`. It's the same, but less confusing. – Some Guy Sep 01 '12 at 12:34
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Do You mean `var body = entire.slice(entire.indexOf("{") + 1, entire.lastIndexOf("}"));` ? – divide by zero Sep 01 '12 at 12:38
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Yes, I do. `substring` and `substr` generally confuse people. `slice` is a lot more memorable. – Some Guy Sep 01 '12 at 12:39
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I will update the answer :) so if people will look for it - they receive better solution. Thanks – divide by zero Sep 01 '12 at 12:40
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If you find a duplicate, then please flag the question as duplicate and if you reuse other answers then also cite them properly. – Felix Kling Sep 01 '12 at 12:40
It wont work for the native functions, just for the user defined functions – GCC Apr 14 '20 at 05:54
Also note: this answer is from 2012, before we could use arrow functions. If you try this on an arrow function it also wont work. Just use `func.toString()` when logging arrow functions – Mettin Parzinski Dec 28 '20 at 14:10
It won't work for functions that have parameters with default values. Example: function(myObj = {}) { console.log('Contents') } – José Manuel Blasco Aug 26 '21 at 11:23

score 10 · Answer 2 · edited Aug 07 '19 at 14:30

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Since you want the text between the first { and last }:

derp.toString().replace(/^[^{]*{\s*/,'').replace(/\s*}[^}]*$/,'');

Note that I broke the replacement down into to regexes instead of one regex covering the whole thing (.replace(/^[^{]*{\s*([\d\D]*)\s*}[^}]*$/,'$1')) because it's much less memory-intensive.

edited Aug 07 '19 at 14:30

Pierre C.

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answered Sep 01 '12 at 12:34

Niet the Dark Absol

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`toString()` instead of `toString` but this is the cleanest answer I've found, thank you ! – kursus Aug 16 '17 at 21:32

score 4 · Answer 3 · answered Sep 05 '13 at 23:57

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NOTE: The accepted answer depends on the interpreter not doing crazy things like throwing back comments between 'function' and '{'. IE8 will happily do this:

>>var x = function /* this is a counter-example { */ () {return "of the genre"};
>>x.toString();
"function /* this is a counter-example { */ () {return "of the genre"}"

answered Sep 05 '13 at 23:57

Ian

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This is not an answer but a comment – Jorge Fuentes González Mar 16 '18 at 13:40

score 3 · Answer 4 · answered Dec 30 '14 at 18:14

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A single-line, short regex example:

var body = f.toString().match(/^[^{]+\{(.*?)\}$/)[1];

If you want to, eventually, eval the script, and assuming the function takes no parameters, this should be a tiny bit faster:

var body = '(' + f.toString() + ')()';

answered Dec 30 '14 at 18:14

K3---rnc

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Turning it into an IIFE statement for evaluation is the most robust solution if the function takes in no arguments. Works with ES6 arrow function syntax too. – Shi Ling May 08 '18 at 09:51

score 2 · Answer 5 · edited Sep 06 '17 at 15:34

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You need something like this:

var content = derp.toString();
var body = content.match(/{[\w\W]*}/);
body = body.slice(1, body.length - 1);

console.log(body); // a(); b(); c();

edited Sep 06 '17 at 15:34

sdgluck

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answered Sep 01 '12 at 12:41

micnic

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How to get function body text in JavaScript?

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