How is it possible that TypeScript does not provide a warning when a function is called with a different number of arguments than what is expected?

I am working on a vanilla JavaScript project in VS Code and have set up jsconfig.json. Here is an example of the code I am using:

/**
 * @param {(arg: string) => void} nestedFunction
 */
function myFunction(nestedFunction) {
  // Some logic here
}

myFunction("text");       // Error is highlighted
myFunction(() => {});     // Error not highlighted due to fewer arguments
myFunction((a, b) => {}); // Error not highlighted due to more arguments and "a" as type "any"

The same issue occurs when using TypeScript with the function defined as:

function myFunction(nestedFunction: (arg: string) => void) {
  // Some logic here
}

VS Code correctly identifies errors when passing the wrong argument type to myFunction (such as a string), but does not flag any issues when passing a function that doesn't match the format of nestedFunction. Why is this?

In case it's relevant, here is the content of my jsconfig.json file:

{
  "compilerOptions": {
    "checkJs": true,
    "target": "ES6"
  }
}

Answer №1

VS Code's support for JavaScript and TypeScript stems from the internal TypeScript Language Features extension, built entirely on TypeScript.

Using a Function with Fewer Arguments than Expected

In some cases, functions with fewer parameters are assignable to functions that require more parameters, as explained in the TypeScript FAQ.

Substitutability is a key concept here, stating that objects can be interchanged if they have similar properties and functionalities.

For instance, using a function with fewer arguments than specified is a common practice in JavaScript coding patterns.

Using a Function with More Arguments than Expected

In TypeScript files, it is generally acceptable to use extra arguments only if those parameters are marked as optional. This ensures compatibility with the caller's perspective.

When callbacks have optional parameters, the function still provides all arguments when invoked, eliminating the need to check for undefined values.

While JavaScript files may not enforce strict parameter rules, upcoming updates like TypeScript 5.4 will offer quick fixes for aligning function calls with their defined signatures.

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