关于数组 Cannot read property '0' of undefined

关于数组Cannot read property ‘0’ of undefined

console.log(keyname) // sale purchase inventory
console.log(keyname[0]) // sale (typeof:string)
console.log(data.sale) // 有值

console.log(data.keyname[0]) //Cannot read property ‘0’ of undefined
正确写法 data[keyname[0]]