5

Is there anyway to convert a string to a condition inside if statement:

example:

var condition = "a == b && ( a > 5 || b > 5)";

if(condition) {
    alert("succes");
}
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4
  • 1
    eval or new Function, but that is a bad idea.... Commented Mar 18, 2019 at 15:29
  • 2
    You can technically, but that is not a good practice - Can you add a bit of context to your question? Why do you need that ? Commented Mar 18, 2019 at 15:29
  • 1
    Possible duplicate of Why is using the JavaScript eval function a bad idea? Commented Mar 18, 2019 at 15:29
  • 24ways.org/2005/dont-be-eval Commented Mar 18, 2019 at 15:47

5 Answers 5

Reset to default
7

A safer alternative to eval() may be Function().

var condition = "4 == 4 && ( 10 > 5 || 9 > 5)";
var evaluate = (c) => Function(`return ${c}`)();

if(evaluate(condition)) {
    alert("succes");
}

Per MDN:

eval() is a dangerous function, which executes the code it's passed with the privileges of the caller. If you run eval() with a string that could be affected by a malicious party, you may end up running malicious code on the user's machine with the permissions of your webpage / extension. More importantly, a third-party code can see the scope in which eval() was invoked, which can lead to possible attacks in ways to which the similar Function is not susceptible.

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1 Comment

Does Function used this way alleviate any of eval()'s issues?
2

You can use new function

let a = 6;
let b = 6
var condition = "a == b && ( a > 5 || b > 5)";

let func = new Function('a','b', `return (${condition})` )

if(func(a,b)) {
    alert("succes");
}

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1 Comment

This one is really great answer. Thank you so much.
1

eval can help, but try to avoid using it

let a = 6;
let b = 6
var condition = "a == b && ( a > 5 || b > 5)";

if (eval(condition)) {
  alert("succes");
}

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3 Comments

Agreed, no use eval if possible.
@MichaelLeVan stackoverflow.com/questions/86513/…
@MichaelLeVan MDN: Do not ever use eval()!
0

use eval :

var condition = "4 == 4 && ( 10 > 5 || 9 > 5)";

if(eval(condition)) {
    alert("succes");
}

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1 Comment

There's a reason eval is shunned.
0

Convert your data and condition to JSON

Then set the condition by hand:

var json = '{"a":20, "b":20, "max":5}'; //Max is your condition
var data = JSON.parse(json);
var a = data.a;
var b = data.b;
var max = data.max;

if(a == b && ( a > 5 || b > 5)) {
    console.log("foobar");
}

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