SOURCE

console 命令行工具 X clear

                    
>
console
var b = 1,c;
if(typeof a !== 'undefined') {
  console.log('a exists!')
} else {
  console.log('a does not exists!')
}

if(typeof b !== 'undefined') {
  console.log('b exists!')
} else {
  console.log('b does not exists!')
}

if(typeof c !== 'undefined') {
  console.log('c exists!')
} else {
  console.log('c does not exists!')
}
//已定义但没有赋值或者没有定义