Dynamically changing a Javascript map statement

Question

I want to evaluate a dynamically created string inside a map function. This is how it should work:

ar arr = [1,2,3,4,5];
var arr2 = arr.map(v=>((v+5)*6));     //Gives  [36, 42, 48, 54, 60]

I have created a string as below:

var s = '((v+5)*6)';
var arr2 = arr.map(v=>eval(s));      // I need something like this to work!

Can you do two things please?

Show how this can be done using eval on the string I have created (or an amended string).
Suggest another (better) method of doing this - I'm sure there is one.
Don't just skip out point 1)

Thanks

Could you add some more context as to why eval is necessary? — Emiel Zuurbier
– Emiel Zuurbier, Commented Dec 25, 2020 at 10:36
Is there a reason why you need to use eval? Even if you’re relying on user input that’s never a good idea. — Terry
– Terry, Commented Dec 25, 2020 at 10:37
The map statement is dynamically created. So the input could be something like this: var cmds = [ [ [ '+', '5' ], [ '*', '6' ] ] ]; I can code the input to form any string. I chose to try a method which took the array above and produces the string: "((v+5)*6))". — jeebee
– jeebee, Commented Dec 25, 2020 at 10:39
Point1: Your eval solution still works. Point2: You should provide more context to get help. — thelonglqd
– thelonglqd, Commented Dec 25, 2020 at 10:46

KooiInc · Accepted Answer · 2020-12-25 11:04:04Z

3

Notwithstanding the fact that eval probably is the wrong way to implement, this would be the way to make it run.

const arr = [1,2,3,4,5];
let s = '((v+5)*6)';
const arr2 = arr.map(eval(`v => ${s}`));
console.log(arr2);

Otherwise you should use or create a parser for the string, like Math.js. Or check this page for ideas.

answered Dec 25, 2020 at 11:04

KooiInc

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4 Comments

jeebee Over a year ago

I made a booboo. Eval is disabled in the environment I am using. So my code would have worked - explains why I was so confused. Thanks for your help and all others who contributed. I will look at one of the other methods you suggested.

KooiInc Over a year ago

;) I'd say it is disabled for a reason. Hope the links in my answer can help then.

jeebee Over a year ago

I can't use the math library. Is there no easier way to do this than to write my own maths expression parser?

jeebee Over a year ago

The order of operations should follow the brackets I have added from inner to outer.

jeebee · Accepted Answer · 2020-12-25 14:03:19Z

0

arr.map(v=>calc(v))

Pass something like this to the map function:

function calc(v){
          if (a=='plus') {return v+n;}
          if (a=='minus') {return v-n;}
          if (a=='multiply') {return v*n;}
          if (a=='divide') {return v/n;}
          if (a=='modulo') {return v%n;}
 }

answered Dec 25, 2020 at 14:03

jeebee

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