I'm writing function that'll return single value from object by key or return empty object if key does not exist.

Code:

//example data:
const obj = {
    data: {test: 'word'},
    str: 'Hi',
    num: 22,
}

const pickOne = <T extends object>(obj: T, key: keyof typeof obj) => ((key in obj) ? obj.key : {})

console.log(pickOne(obj, 'data')) //should print {test: 'word'}

My problem is that I get this error: Property 'key' does not exist on type 'T'. I'm new to typescript but after reading docs I was sure that keyof typeof would allow any key in obj.

Any help would be appreciated, thanks.

CodePudding user response：

This:

obj.key

gets the property named "key" on obj, which may not exist.

Where this:

obj[key]

Gets the property name in the variable key.

Which works without type error: See playground

CodePudding user response：

Your obj has 3 keys:

• data
• str
• num

When you write obj.key you are telling the interpreter to find a key called key, which does not exist.

On the other hand you can access these keys by its name using brackets:

• const key = "data"
• obj[key] // Should return {test: 'word'} , which is equivalent to:
• obj["data"] // Should return {test: 'word'}

Moral of the story:

1. The dot-notation is to access existing keys in a given object
   • If the key exists it works. Otherwise, it errors out
2. TypreScript also allows accessing these keys by name
   • If the key exists it works. Otherwise, doesn't error out but returns undefined

Working example:

const obj = {
    data: {test: 'word'},
    str: 'Hi',
    num: 22,
}

const key = "data"
console.log(obj[key])
console.log(obj["data"])
console.log(obj["key"])