Check out this comparison between http://jsperf.com/in-vs-member-object-access
The question at hand is why the execution of if ('bar' in foo) {} is noticeably slower than if (foo.bar !== undefined) {}?
Check out this comparison between http://jsperf.com/in-vs-member-object-access
The question at hand is why the execution of if ('bar' in foo) {} is noticeably slower than if (foo.bar !== undefined) {}?
When using 'foo.bar !== undefined', it only checks those two values for a match.However, when you use the expression 'bar' in foofoo to check if bar is present.
If you're interested, here's some enlightening information from the world of Ecma-script!
Understanding the 'in' Operator
The process of evaluating the expression RelationalExpression: RelationalExpression in ShiftExpression can be broken down into steps:
1. Evaluate the RelationalExpression.
2. Get the value of Result(1).
3. Evaluate the ShiftExpression.
4. Get the value of Result(3).
5. If Result(4) isn't an object, throw a TypeError exception.
6. Convert Result(2) to a string.
7. Use the [[HasProperty]] method of Result(4) with parameter Result(6).
8. Return Result(7).Diving into the Strict Does-not-equal Operator (!==)
When evaluating EqualityExpression : EqualityExpression !== RelationalExpression, follow these steps:
1. Evaluate the EqualityExpression.
2. Obtain the value of Result(1).
3. Evaluate the RelationalExpression.
4. Get the value of Result(3).
5. Compare Result(4) === Result(2). (Refer below.) 6. If this comparison yields true, return false; otherwise, return true.
It is indeed perplexing that "bar" in foo seems to be slower than foo.bar.
The reason behind the difference in speed could possibly be attributed to the fact that the in operator has not received as much optimization attention from Just-In-Time (JIT) engineers compared to the widely used foo.bar syntax.
In cases like your jsperf test, where the property exists directly on foo itself and not within a prototype chain, it logically follows that 'bar' in foo should be just as fast, if not faster, than foo.bar !== undefined. This is because the in operator can determine existence without evaluating the property value.
It's possible that both the V8 engine and the SpiderMonkey engine are capable of identifying that the code involving foo.bar serves no practical purpose and can be optimized out completely. In reality, the benchmark may not be reflecting any meaningful operations.
Although current engines may not yet optimize "bar" in foo effectively, advancements in optimization techniques are constantly evolving. It's only a matter of time and priorities before such improvements are implemented.
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