I recently came across a code snippet (a joke) where the test() method of JavaScript's RegExp object was used with an empty object as an argument. Here's the code:

console.log(new RegExp({}).test('mom')); // true
console.log(new RegExp({}).test('dad')); // false

Can someone explain why is it happens?

I recently came across a code snippet (a joke) where the test() method of JavaScript's RegExp object was used with an empty object as an argument. Here's the code:

console.log(new RegExp({}).test('mom')); // true
console.log(new RegExp({}).test('dad')); // false

Can someone explain why is it happens?

• javascript
• regex
• object

Share Improve this question Follow asked Mar 20, 2024 at 23:02 AgorrecaAgorreca 6861717 silver badges3131 bronze badges 3

• 1 try console.log(new RegExp({})); and see if you can figure it out – Jaromanda X Commented Mar 20, 2024 at 23:08
• It was explained in a ment on the Twitter post. – Dave Newton Commented Mar 20, 2024 at 23:13
• dad's get no love! :( – Ishan Commented Mar 21, 2024 at 2:00

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 8

This is a curious fact. RegExp constructor accepts a string as its first argument. Since you are passing {} it gets coerced to string, and the coercion of an object is the literal string [object Object].

By a fortuitous coincidence, the square brackets have a precise meaning in a regular expression, and it means "one of these characters of the set".

Thus, the regular expression is equal to [objectO ]. In other words, your code is equal to:

console.log(new RegExp('[object Object]').test('mom'));

which is equal to:

console.log(new RegExp('[objectO ]').test('mom'));

which means: tests true if any of these characters is present: o, b, j, e, c, t, O, space. mom satisfies this condition, while dad doesn't.

The RegExp constructor casts the first argument to a string. An empty object bees

new RegExp('[object Object]')

Given this represents a regexp character class, essentially

/[objectO ]/

"mom" passes because it contains an "o".