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Why isn't VS Code providing autocomplete for methods on the return type of my JavaScript function?

March 27, 2023

I am working on a web development project using Visual Studio Code and JavaScript. In my code, I have a function that creates a new div element and assigns it to a variable named newDiv. Here is the code:

function createNode(tag,id) {
    let newNode = document.createElement(tag)
    newNode.id = id;
    return newNode;
}

let newDiv = createNode("div", "identificador1");

However, when I type "newDiv." in my code editor, VS Code does not offer any methods or properties for newDiv, even though it is a div element.

I expected to see a list of available methods, such as classList, getAttribute, and appendChild, but nothing appears.

I have tried the following solutions:

Restarting VS Code
Checking that I am typing the period (".") correctly
Checking there are no typos.
Explicitly specifying the type of newDiv as HTMLDivElement works, and VS Code starts to offering methods, but the word "as" is causing an Uncaught SyntaxError. like this:
```
let newDiv = createNode("div", "identificador1") as HTMLDivElement;
```

None of these solutions have worked for me. Can someone please help me understand why VS Code is not offering methods for newDiv, and how I can fix this issue?

>Solution :

You haven’t done anything to tell static analysis about the expected types of the inputs for your function. In VS Code, you can do this for JavaScript with JS Doc annotations:

/**
 * @param tag {keyof HTMLElementTagNameMap}
 * @param id {string}
 */
function createNode(tag,id) {
    let newNode = document.createElement(tag)
    newNode.id = id;
    return newNode;
}

The static analysis of VS Code powered by TypeScript will be able to infer the return type now based on the input type annotations and the body of the function. (It will suggest an intersection type of all type values in HTMLElementTagNameMap since it can’t do better without generics). If you want the HTMLDivElement type exactly, you will have to hint it using a /**@type {HTMLDivElement}*/ let newDiv = createNode(...);.

You could get better return type inference if you could use better generics (which TypeScript has) in JS Doc. It seems that VS Code doesn’t support TypeScript generics in type annotations very deeply right now. I tried the following just as a hypothesis and such syntax doesn’t seem to be supported:

/**
 * @template T {keyof HTMLElementTagNameMap}
 * @param tag {T}
 * @param id {string}
 * @returns {HTMLElementTagNameMap[T]}
 */
function createNode(tag,id) {
    let newNode = document.createElement(tag)
    newNode.id = id;
    return newNode;
}

Your attempt to use the as (type assertion) keyword didn’t work because that’s not JavaScript syntax. It’s TypeScript syntax. You can use type annotations in JavaScript with JS Doc using @type {...}.