This particular function is quite unconventional. Let's delve into the code to understand it better.
Firstly, consider the following example:
const something = ('' as any) as boolean
// Here, we're essentially tricking Typescript by declaring 'something' as a boolean type, even though its value is actually a string!
The (X as any) as Y approach is a workaround that allows you to inform Typescript that something is different from what it appears to be. This manipulation of types gives you total control over the type system, potentially leading to bugs like in the above case where an empty string is labeled as a boolean.
Now, let's move on to the next technique:
const isTruthy: <T>( x: T | false | undefined | null | '' ) => x is T
// Feature not yet implemented
With this declaration, Typescript is informed that "this is a function which can accept any parameter type, and if it returns true, then the parameter is definitely truthy". The concept uses Type guards with the return type being 'x is T'. This method enables you to verify non-falsy values accurately:
const a: any[] | null | undefined = [];
if(isTruthy(a)) {
// 'a' is considered as an array.
}
An easy way to implement the 'isTruthy' function is by leveraging the 'Boolean' function:
type Falsy = null | undefined | false | 0 | ''
function isTruthy<T>(value: T | Falsy): value is T {
return Boolean(value);
}
So, what's the purpose behind all this?
Let's revisit your original example:
const check = (Boolean as any) as <T>(
a: T | false | undefined | null | ''
) => a is T;
- In this code snippet, 'Boolean' is referred to as a value, namely the Boolean Function.
- It's casted to 'any', and then casted again as '<T>( x: T | false | undefined | null | '' ) => x is T'
Therefore, this piece of code simply creates an alias for the 'Boolean' function and informs Typescript that it acts as a type guard. Whenever 'check(a)' returns true, it signifies that 'a' is indeed non-falsy.
Alternatively, you could directly check the value itself:
type Falsy = null | undefined | false | 0 | ''
const a: string | Falsy = 'test'
if (a) {
// 'a' is inferred as just "string"
}
TL;DR
Your 'check' function employs some clever manipulations to utilize the 'Boolean' function for tasks that a simple 'if()' statement can accomplish effortlessly.
Just assess the truthiness of a value using 'if(value)' and proceed.
<edit>
I inadvertently missed addressing a crucial aspect of the query:
Could someone please elucidate how this function operates with array inputs? A step-by-step breakdown of the syntax in this scenario would be beneficial. Thank you.
In such instances, your 'check' function could be paired with the 'filter' method for precise type inference:
const arr = ['a', false, 123, 0, '', null, undefined];
arr.filter(Boolean).forEach(value => {
// Although 'filter' guarantees non-falsy values, 'value' might still appear falsy.
});
const check = (Boolean as any) as <T>(
a: T | false | undefined | null | ''
) => a is T;
arr.filter(check).forEach(value => {
// 'value' can be either a string, number, or true
});
Since Typescript doesn't inherently recognize the proficiency of the Boolean function as a guard, this illustrates a useful application for your 'check' function, albeit renaming it to 'isTruthy' would be more apt.