How to do math on strings in JavaScript?

Question

var string = "2+3-1";
document.write(string);

In the code above, I have a string with digits and plus and minus operators. I want to calculate the string when i want to display it. I mean, when i want to dispaly, it should calculate and show 4. I tried to convert them to number but gives error.

Maheer Ali · Accepted Answer · 2020-03-25 14:31:55Z

5

You can use the built in method eval()

Note: Using eval() is never recommended. You should use some kind of external library like https://mathjs.org/

var string = "2+3-1";
document.write(eval(string));

edited Mar 25, 2020 at 14:31

answered Mar 25, 2020 at 14:27

Maheer Ali

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3 Comments

Aplet123 Over a year ago

There should be a warning that this is VERY unsafe if the string is not hardcoded. For a safer alternative purely for doing math, a library like mathjs should be used.

Maheer Ali Over a year ago

@Aplet123 Thanks. I added a note for that.

Elias Achilles Over a year ago

Thanks for the answer. What causes that eval() is never recommended?

Kosh · Accepted Answer · 2020-03-25 22:01:35Z

1

If you're using plus and minus only, you might do something like this:

var string = "2+3-1";
document.write(string);

var result = string.match(/([-+]?\d+)/g).reduce((a, e) => a- -e, 0);
document.write(' = ' + result)

edited Mar 25, 2020 at 22:01

answered Mar 25, 2020 at 14:50

Kosh

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4 Comments

Elias Achilles Over a year ago

Awesome! One question, how did you convert the string to number?

Kosh Over a year ago

@EliasAchilles, What do you mean?

Elias Achilles Over a year ago

I mean, in the code above, you cannot do math on string. The variable result is still string and it shouldn't do the calculation, as long as i know. But it did.

Kosh Over a year ago

@EliasAchilles, var result is a number calculated via match and reduce methods, not a string. Probably you're confused with the formatting of my code.

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