JS - Best way to cast string value to number, boolean or string?

Question

As the title says, I'm trying to build a function that given a string value as its input, it returns the value casted properly.

So far I got this, I think it looks ugly and have the feeling it can be solved in a better way.

Examples:

const smartCast = function (value) {
  if (value === 'true') {
    return true;
  }
  if (value === 'false') {
    return false;
  }
  if (!isNaN(value)) {
    return Number(value);
  }

  return value;
};

console.log(smartCast('ahoy!') === 'ahoy!');
console.log(smartCast('hello there') === 'hello there');
console.log(smartCast('412') === 412);
console.log(smartCast('71.3') === 71.3);
console.log(smartCast('true') === true);
console.log(smartCast('false') === false);

Let me know if there is a case I'm not taking into account.

except for booleans, if you use a ``` == ``` for checking this equality, you'll be able to get rid of the function altogether. Also instead of passing the string to smartCast why not pass the right hand variable to smartCast and have the function like ``` const smartCast2 = function (value) { return String.value(value); }; ``` ? — dixitk13
– dixitk13, Commented Sep 29, 2018 at 5:21
It would be better if you convert both to a common type and then determine the equality. — Shankar Shastri
– Shankar Shastri, Commented Sep 29, 2018 at 5:52
if (value === 'true') { return true; } if (value === 'false') { return false; } can be combined as if(value==='true' || value==='false'){return value==='true'} — Just code
– Just code, Commented Sep 29, 2018 at 6:07

Alexandre Borela · Accepted Answer · 2018-09-29 12:32:02Z

2

function cast(value) {
  if (typeof value !== 'string')
    return value

  // Ignore values that would start an array or object.
  if (/^\s*[{\[]/.test(value))
    return value

  try {
    // Try to parse as some value.
    return JSON.parse(value)
  } catch(e) {
    // If there's an error, we will assume it is just
    // a string.
  }

  return value
}

console.log(typeof cast('false')) // boolean
console.log(typeof cast('true')) // boolean
console.log(typeof cast('1')) // number
console.log(typeof cast('3.14')) // number
console.log(typeof cast('abc')) // string
console.log(typeof cast('[1,2,3]')) // string
console.log(typeof cast('{"abc":1}')) // string

edited Sep 29, 2018 at 12:32

answered Sep 29, 2018 at 8:42

Alexandre Borela

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3 Comments

Bergi Over a year ago

…or drop the try/catch and simply feed valid JSON instead

Alvaro Castro Over a year ago

This input "[1, 2, 3]" would return an array instead of a string. Anyway I like this answer so far.

Alexandre Borela Over a year ago

Added a check for objects and arrays.

Ebuall · Accepted Answer · 2018-09-29 13:19:06Z

0

Take a look at empty string case and date https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN#Examples

answered Sep 29, 2018 at 13:19

Ebuall

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