How to check variable is undefined in javascript
Q：which is better between === undefined or typeof variable === 'undefined'
A：typeof is aways safety
https://stackoverflow.com/questions/4725603/variable-undefined-vs-typeof-variable-undefined
Q：which is better between === undefined or typeof variable === 'undefined'
A：typeof is aways safety
https://stackoverflow.com/questions/4725603/variable-undefined-vs-typeof-variable-undefined