JS object and number addition

Question

{} + 5 === 5
5 + {} === '5[object Object]'

How is the first expression {} + 5 === 5 calculated?

The second expression 5 + {} === '5[object Object]' is expected result.

===== Edit ====

({}) + 5 === '[object Object]5'

Which might be to say: {} in first expression was ignored as the question comment says.

This is surely a dupe, but I don't think it has anything to do with JSFuck — CertainPerformance, Feb 23 '19 at 09:30
@PraveenKumarPurushothaman that does not at all explain that the first "object" is interpreted as an empty code block — jonrsharpe, Feb 23 '19 at 09:30
@CertainPerformance the empty code block is ignored, so it's a unary plus; that's why the result is just `5` — jonrsharpe, Feb 23 '19 at 09:34

abadalyan · Accepted Answer · 2019-02-24T08:21:42.007

2

The {} at the beginning of a line is considered a code block rather than an object literal. Thus {} + 5 is not considered a binary addition between two values and evaluates to +5, unary + operator applied to 5.

When {} is placed inside round brackets it turns into object literal and the entire expression evaluates to '[object Object]5'

More details on this gotcha can be found here

edited Feb 24 '19 at 08:21

answered Feb 23 '19 at 14:05

abadalyan

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score 1 · Answer 2 · answered Feb 23 '19 at 13:32

In the following snippet, both expressions are converted to strings, so you get the following results, which is expected:

[object Object]5
5[object Object]

The reason this is happening is because + can't be addition between numbers since {} can't be cast to a number. Instead + is considered string concatenation here and both operands get converted to strings.

console.log({} + 5 === '[object Object]5')
console.log(5 + {} === '5[object Object]')

JS object and number addition

2 Answers2