为什么 performance.hasOwnProperty('getEntries') 返回 false 而 typeof performance.getEntries 返回 function？

Question

Window property performance has a function call getEntries to retrieve all performance entries. which works on all modern browsers but doesn't work on a few older browsers like Safari 10. To add a check of working browsers....

If we try to verify using performance.hasOwnProperty('getEntries') , It always return false. But it works if use typeof performance.getEntries === 'function' .

Would like to understand the logic behind it.

Answer 1

performance does not has own property “getEntries”， this property is owned by prototype of Performance, which is the constructor of performance object.

when you use performance.getEntries(), actually Performance.prototype.getEntries() is called.