# Tree Shaking

• 摇树相关
• the codes that unused export that should be dropped

# sideEffect

• sideEffects:true tells webpack your module surely safe , ignore all ununsed codes and do not build into bundle (把未使用但无副作用的模块一并删除)

• how to know

• sure do NOT modify the properties of window

• replicated native object methods

• etc...