I am dealing with a function that takes in multiple string arguments and one final argument of a complex type, which is called Expression.
This particular code looks like this in JavaScript:
function layerProp(...args) {
const fields = args.slice(0, -1)
const fallbackExpression = args.slice(-1)[0]
My question is, how can I create a type definition for this function?
You have the option to utilize this specific type:
function layerProp(...args: [...string[], Expression]) {}
Nevertheless, it does not cover typing for the slice, and a simple solution is to assert the last element as Expression like so:
function layerProp(...args: [...string[], Expression]) {
const fields = args.slice(0, -1);
const fallbackExpression = args.slice(-1)[0] as Expression;
}
Here is how you can use it:
type Expression = {
a: string;
}
layerProp('a', 'b', 'c', { a: 'sd' });
layerProp('a', 'b', 'c', { a: 'sd' }, 'a'); // expected error
layerProp('a', 'b', 'c', 'a'); // expected error
When specifying the function argument, consider using ...args: string[]. The implementation of fallbackExpression in the function is a simple string, indicating that it might be more appropriate to utilize an interface instead of a type.
function defineProp(...args: string[]) {
const fields = args.slice(0, -1)
const fallbackExpression = args.slice(-1)[0]
// additional code here
}
To define a union type, you can use the following syntax:
(string | Expression)[]
With this defined, your code can now be written as follows:
function layerProp(...args: (string | Expression)[]) {
const fields = args.slice(0, -1)
const fallbackExpression = args.slice(-1)[0]
// continue with the remaining code
}
When dealing with a single complex Expression object and multiple strings, one helpful suggestion is to rearrange the argument positions to allow for more flexibility. This alternative approach would look like:
function propLayer(expression: Expression, ...values: strings[]) {
// ... custom code here
}
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