make Strings.startsWith to support ${Prefix}${string}

[zhujinxuan]

lib Update Request

Sometimes, we might make a more specific definition on string type to avoid confusion in programming. For example:

const statePrefix = `State/` as const
type State = `${typeof statePrefix}${string}`
const customerIdPrefix = `Customer/` as const
type CustomerId = `${typeof customerIdPrefix}`

In this way, we can distinguish state and customerId, rather than using string everywhere. For example, we can clearly know a structure of a dictionary like

// Avoid confusion of Map<string, Map<string, boolean>>
const IsActiveCustomerMap = new Map<State<Map<CustomerId, boolean>>>()

So, I suggest adding startsWith definition to understand literal types. I would suggest this definition added to the library in addition to the original definition:

    startsWith<T extends string>(searchString: T, position?: 0): this is `${T}${string};

This definition can help us simplify the specific string definition.

Configuration Check

My compilation target is ES2015 and my lib is the default.

Missing / Incorrect Definition

Add to Strings:

    startsWith<T extends string>(searchString: T, position?: 0): this is `${T}${string};

Sample Code

function mkPrefix<T extends string>(prefix: T, key: string): `${T}${string}` {
  if (key.startsWith(prefix)) return key;
  return `${prefix}${key}`
}

Documentation Link

[zhujinxuan]

I can make the PR if this change is acceptable.

[MartinJohns]

Sounds like a duplicate of #46958.

[zhujinxuan]

Yes, it is a duplicate. I will close the issue