TypeScript:HTMLElementのキャスト

Question

TypeScriptでキャストする方法を知っている人はいますか？

私はこれをやろうとしています:

var script:HTMLScriptElement = document.getElementsByName("script")[0];
alert(script.type);

しかし、それは私にエラーを与えています:

Cannot convert 'Node' to 'HTMLScriptElement': Type 'Node' is missing property 'defer' from type 'HTMLScriptElement'
(elementName: string) => NodeList

正しい型にキャストしない限り、script要素の'type'メンバーにアクセスできませんが、これを行う方法がわかりません。ドキュメントとサンプルを検索しましたが、何も見つかりませんでした。

Answer 1

TypesScriptはキャストをサラウンドするために '<>'を使用するので、上記は次のようになります。

var script = <HTMLScriptElement>document.getElementsByName("script")[0];

.

しかし、残念ながらあなたはできません：

var script = (<HTMLScriptElement[]>document.getElementsByName(id))[0];

.

エラー

Cannot convert 'NodeList' to 'HTMLScriptElement[]'

.

しかしあなたはできる：

(<HTMLScriptElement[]><any>document.getElementsByName(id))[0];

.

Answer 2

のようになりました。 lib.d.ts ファイルは、呼び出しの正しい型を返す特殊なオーバーロードシグネチャを使用します getElementsByTagName.

これは、型アサーションを使用して型を変更する必要がなくなることを意味します:

// No type assertions needed
var script: HTMLScriptElement = document.getElementsByTagName('script')[0];
alert(script.type);

Answer 3

常にタイプシステムを使ってハックすることができます。

var script = (<HTMLScriptElement[]><any>document.getElementsByName(id))[0];

.

Answer 4

キャストを入力しないでください。絶対に。型ガード：を使用してください

const e = document.getElementsByName("script")[0];
if (!(e instanceof HTMLScriptElement)) 
  throw new Error(`Expected e to be an HTMLScriptElement, was ${e && e.constructor && e.constructor.name || e}`);
// locally TypeScript now types e as an HTMLScriptElement, same as if you casted it.

.

あなたの仮定が間違っていることが判明したときに、コンパイラはあなたのための作業を行い、エラーを得ることができます。

この場合は過剰に見えるかもしれませんが、後で戻ってきてセレクターを変更してください。たとえば、DOMに欠けているクラスを追加してください。

Answer 5

To end up with:

• an actual Array object (not a NodeList dressed up as an Array)
• a list that is guaranteed to only include HTMLElements, not Nodes force-casted to HTMLElements
• a warm fuzzy feeling to do The Right Thing

Try this:

let nodeList : NodeList = document.getElementsByTagName('script');
let elementList : Array<HTMLElement> = [];

if (nodeList) {
    for (let i = 0; i < nodeList.length; i++) {
        let node : Node = nodeList[i];

        // Make sure it's really an Element
        if (node.nodeType == Node.ELEMENT_NODE) {
            elementList.push(node as HTMLElement);
        }
    }
}

Enjoy.

Answer 6

Just to clarify, this is correct.

Cannot convert 'NodeList' to 'HTMLScriptElement[]'

as a NodeList is not an actual array (e.g. it doesn't contain .forEach, .slice, .push, etc...).

Thus if it did convert to HTMLScriptElement[] in the type system, you'd get no type errors if you tried to call Array.prototype members on it at compile time, but it would fail at run time.

Answer 7

Updated example:

const script: HTMLScriptElement = document.getElementsByName(id).item(0) as HTMLScriptElement;

Documentation:

TypeScript - Basic Types - Type assertions

Answer 8

This seems to solve the problem, using the [index: TYPE] array access type, cheers.

interface ScriptNodeList extends NodeList {
    [index: number]: HTMLScriptElement;
}

var script = ( <ScriptNodeList>document.getElementsByName('foo') )[0];

Answer 9

Could be solved in the declaration file (lib.d.ts) if TypeScript would define HTMLCollection instead of NodeList as a return type.

DOM4 also specifies this as the correct return type, but older DOM specifications are less clear.

See also http://typescript.codeplex.com/workitem/252

Answer 10

Since it's a NodeList, not an Array, you shouldn't really be using brackets or casting to Array. The property way to get the first node is:

document.getElementsByName(id).item(0)

You can just cast that:

var script = <HTMLScriptElement> document.getElementsByName(id).item(0)

Or, extend NodeList:

interface HTMLScriptElementNodeList extends NodeList
{
    item(index: number): HTMLScriptElement;
}
var scripts = <HTMLScriptElementNodeList> document.getElementsByName('script'),
    script = scripts.item(0);

Answer 11

I would also recommend the sitepen guides

https://www.sitepen.com/blog/2013/12/31/definitive-guide-to-typescript/ (see below) and https://www.sitepen.com/blog/2014/08/22/advanced-typescript-concepts-classes-types/

TypeScript also allows you to specify different return types when an exact string is provided as an argument to a function. For example, TypeScript’s ambient declaration for the DOM’s createElement method looks like this:

createElement(tagName: 'a'): HTMLAnchorElement;
createElement(tagName: 'abbr'): HTMLElement;
createElement(tagName: 'address'): HTMLElement;
createElement(tagName: 'area'): HTMLAreaElement;
// ... etc.
createElement(tagName: string): HTMLElement;

This means, in TypeScript, when you call e.g. document.createElement('video'), TypeScript knows the return value is an HTMLVideoElement and will be able to ensure you are interacting correctly with the DOM Video API without any need to type assert.

Answer 12

var script = (<HTMLScriptElement[]><any>document.getElementsByName(id))[0];