Typescript tips

Accessing objects

It is a common javascript pattern to check if an object property exists by trying to access it:

if (obj[key]) {
    // this key might not exist on obj so there is a typescript error.
}

Typescript knows that the key might not be on the object, so it throws an error, even though we wanted to use its absence as a check in our code. We could force typescript to believe that the key is on the object using as:

if (obj[key as keyof typeof obj]) {
    // this key might not exist on obj, but now typescript believes that it does.
}

as lets you set the type of key to whatever type we assign instead of typescript inferring the type.
keyof makes a union type out of all the keys of an object type.
typeof refers to the type of a variable or property, in this case obj.

There are cleaner ways to check if a key exists on an object.
We can use the in operator. in checks if an object or its prototype chain has a property with a name. Many times this check can narrow the type of key.

if (key in obj) {
    // this key exists on obj and typescript is happy
}

Or use the isKey @terascope/utils function. This will always narrow the types. Within the code block, key will be typed as a keyof typeof obj, not just any string, number, or symbol:

if (isKey(obj, key)) {
    // this key exists on obj and typescript is happy
    obj[key] // key is a 'keyof typeof obj'
}