JavaScript - why does JSON.parse(['123']) return 123?

JSON.parse() expects a string and not an array.

However, when we pass an array or any other non-string value, the method automatically converts it to a string and continues instead of throwing an error immediately.

The string representation of an array consists of its values separated by commas:

xxxxxxxxxx

console.log(String(['123']));        // '123'

console.log(String(['123abc']));     // '123abc'

console.log(String(['123', '123'])); // '123,123'

Notice that ['123'] and [123] both convert to the same string: '123'.

So what you're doing is basically:

xxxxxxxxxx

JSON.parse('123');

JSON.parse('123abc');

JSON.parse('123,123');

The last two are not valid JSON, so JSON.parse() throws an error in both cases.

The first one - 123 is a Number literal and therefore valid JSON, representing itself.

That's why JSON.parse('123') (and by extension JSON.parse(['123'])) returns the numeric value 123.

References

1. JSON.parse() - JavaScript | MDN