Javascript - Variable in function name, possible?

Question

i hope this question is not too simple, but i have no idea :(

How can i start a function with a var in the function name?

For example ...

my functions

function at_26();
function at_21();
function at_99();

start the function

var test_id = 21;   
at_'+test_id+'();   // doesn't work

I hope somebody can help me.

Thanks in advance! Peter

Why do you need that? You could create a function called at() and pass your number to it as an argument. — jwueller
– jwueller, Commented Sep 17, 2010 at 8:03

Quentin · Accepted Answer · 2010-09-17 08:02:10Z

102

Store your functions in an object instead of making them top level.

var at = {
    at_26: function() { },
    at_21: function() { },
    at_99: function() { }
};

Then you can access them like any other object:

at['at_' + test_id]();

You could also access them directly from the window object…

window['at_' + test_id]();

… and avoid having to store them in an object, but this means playing in the global scope which should be avoided.

answered Sep 17, 2010 at 8:02

Quentin

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5 Comments

colithium Over a year ago

I'm more curious as to why someone would be creating 100 functions that differ by only an index...

jonschlinkert Over a year ago

Tests. Let's say you have a utilities library that does something like transform paths. You might have an array of paths to against which to test each method. This is an easy way to add utilities to the library without duplicating code to run the tests for each new method.

TheSatinKnight Over a year ago

In my case, they don't. This is just an example of how they might differ. Some differ only in a slight handling requirement for the data being manipulated and it's much simpler to categorize them and use the category to jump between handlers. All sorts of interesting uses.

slebetman Over a year ago

@jonschlinkert In which case they should create an array of functions (the functions could even be anonymous) not hundreds of functions. See Express.js route/middleware definitions for how this should be done properly

Ac Hybl Over a year ago

@colithium In my case, I can use this for "add", "multiply", "divide" etc . . .

JeanHuguesRobert · Accepted Answer · 2010-09-17 22:25:59Z

31

You were close.

var test_id = 21
this['at_'+test_id]()

However, what you may want:

at = []
at[21] = function(){ xxx for 21 xxx }
at[test_id]()

answered Sep 17, 2010 at 22:25

JeanHuguesRobert

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Comments

Nik · Accepted Answer · 2010-09-17 13:27:31Z

3

You can also try

function at_26(){};
function at_21(){};
function at_99(){};

var test_id = 21;   
eval('at_'+test_id+'()');

But use this code if you have very strong reasons for using eval. Using eval in javascript is not a good practice due to its disadvantages such as "using it improperly can open your script to injection attacks."

edited Sep 17, 2010 at 13:27

answered Sep 17, 2010 at 8:07

Nik

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6 Comments

jwueller Over a year ago

No! Do not use eval() in this case! There are much better solutions here!

Nik Over a year ago

@elusive : Thats correct, better solutions always be used. I have just given alternative solution. Hope you don't mind it.

jwueller Over a year ago

Since not everyone necessarily knows the evilness of eval(), you should provide this information in your post. Nobody should use this solution unless he has a very good reason and is exactly knowing what he is doing. eval() has its valid uses but this is not one of them.

Nik Over a year ago

@elusive: Thanks for advice. I have added disclaimer now. Please check.

Hybrid web dev Over a year ago

Never. Ever. EVER use eval. Ever. There's no valid use cases for it. If you think you need it, you need to re-evaluate what you're doing.

|

bigdave · Accepted Answer · 2015-05-06 04:45:59Z

3

There is a better way then the window object - which is NOT friendly in firefox - use "self" instead - so in the example posted by Quentin it looks like this:

self['at_' + test_id]();

answered May 6, 2015 at 4:45

bigdave

392 bronze badges

Comments

NVRM · Accepted Answer · 2021-11-05 20:46:10Z

An example to pass an array of params to those composed functions, .

/* Store function names and match params */
let at = {
    at_26 : (a,b,c) => at_26(a,b,c),
    at_21 : (a,b,c) => at_21(a,b,c),
    at_99 : (a,b,c) => at_99(a,b,c),
    at_om : (a,b,c,d,e) => at_om(a,b,c,d,e)
}

/* Dynamic function router: name + array of Params */
function dynFunc(name, arrayParams){
  return at[name](...arrayParams)
}

/* Usage examples */ 
dynFunc(`at_${99}`, ["track001", 32, true])
dynFunc("at_" + "om", ["track007", [50, false], 7.123, false, "Bye"])


/* In the scope */
function at_99(a,b,c){
  console.log("Hi! " + a,b,c)
  console.log(typeof(a), typeof(b), typeof(c))
}
function at_om(a,b,c,d,e){
  console.log("Hi! " + a,b,c,d,e)
  console.log(typeof(a), typeof(b), typeof(c), typeof(d), typeof(e))
}

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