Odd behaviour when testing for equality in object literals

Question

When testing for equality in object literals, {} === {} returns false (so does [] === []). However, if you write a function to perform the same test and pass an object literal as a parameter, the comparison returns true.

function foo(value) {
    return value === value
} 

foo({}) //Returns true!

Is anyone able to explain this behaviour?

`{}` are two different objects in `{} === {}` but `value` is the same single object. — Mamun, Nov 20 '19 at 00:25

woat · Accepted Answer · 2019-11-20T00:30:52.550

2

The simple answer to this is because the expression {} === {} is taking two object literals, while in the function foo you are taking a singular value and comparing it against itself. In fact, foo will always return true (see edit).

A better way to look at this would be:

const a = {}
const b = {}

console.log(a === b) // false, symmetric to {} === {}
console.log(a === a) // true, symmetric to (val) => val === val

edit: Ryan did point out that when passed NaN, it would short-circuit to false as defined in 11.9.3c.

edited Nov 20 '19 at 00:30

answered Nov 20 '19 at 00:21

woat

431
3
8

That last part is untrue, foo will return false if passed NaN. – Ryan Kavanagh Nov 20 '19 at 00:26

Odd behaviour when testing for equality in object literals

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