JavaScript value not evaluated correctly when in a variable, but works correctly as a string

Question

I am not sure why this evaluation differs when it is only in a variable, versus a string. I don't see any logic.

const numRegex = /hundred|thousand|million|billion|trillion/ig;

const isNum = string => numRegex.test(string)


var word = 'hundred';
console.log('isNum with string:', isNum('hundred')); // true
console.log('isNum with variable:', isNum(word));    // false
console.log('words are equal:', word === 'hundred'); // true

The issue is the order, not whether it's a variable or not. You can copy the first console log, and will get the same results. You are reusing the regex, and due to the `g` flag, the regex' `lastIndex` gets updated, and stays changed in any subsequent calls. — ASDFGerte, Dec 23 '19 at 22:17

Anonymous · Answer 1 · 2019-12-23T22:22:46.737

isNum is returning false when it's called a second time on the same string. Change the order and see the same thing:

const numRegex = /hundred|thousand|million|billion|trillion/ig;

const isNum = string => numRegex.test(string)


var word = 'hundred';
console.log('isNum with variable:', isNum(word));    // true
console.log('isNum with string:', isNum('hundred')); // false
console.log('words are equal:', word === 'hundred'); // true

The g flag remembers where the last match was. Remove it to fix the problem:

const numRegex = /hundred|thousand|million|billion|trillion/i;

Mozilla talks more about this:

The sticky flag indicates that the regular expression performs sticky matching in the target string by attempting to match starting at RegExp.prototype.lastIndex.

JavaScript value not evaluated correctly when in a variable, but works correctly as a string

1 Answers1