Adding Empty Array With Empty Object Yields Different Results When Flipping Operands

Best way to describe my question is to illustrate by example : # node v4.3.1 > {} + [] 0 > [] + {} '[object Object]' > [] + {} === {} + [] true I have managed to find a W

Solution 1:

Case {} + []: JavaScript thinks {} is an empty block statement and it's just ignored. Then + [] -> + '' -> + 0 -> 0. The empty block statement is converted to object literal this way: ({}), so the fixed version should be: ({}) + [].

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Case [] + {}: [] + {} -> '' + '[object Object]' -> "[object Object]".

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Case [] + {} === {} + []: [] + {} === {} + [] -> '' + '[object Object]' === '[object Object]' + '' -> '[object Object]' === '[object Object]' -> true

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Notice:

• An array is transformed to a primitive value using toString() method, which is actually an join(','). [] is transformed to a primitive with [].join(',') -> ''
• The regular Object is transformed to a primitive value using toString() too, which by default returns [object Object]
• <value> + <value2> operator tries to transform it's operands to primitive values. Then tries to concatenate the result of transformation
• + <value> operator transforms the operator to primitive value, then the result is converted to a number

Read more about addition and equality operators.