Manuscrit:casting d'un élément HTML

Question

Est-ce que quelqu'un sait comment caster en TypeScript ?

J'essaie de faire ceci :

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

mais ça me donne une erreur :

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

Je ne peux pas accéder au membre 'type' de l'élément de script à moins de le convertir dans le type correct, mais je ne sais pas comment faire cela.J'ai cherché dans la documentation et les échantillons, mais je n'ai rien trouvé.

Answer 1

Typescript utilise '<>' à couler surround, de sorte que ce qui précède devient:

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

Cependant, malheureusement, vous ne pouvez pas faire:

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

vous obtenez l'erreur

Cannot convert 'NodeList' to 'HTMLScriptElement[]'

Mais vous pouvez faire:

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

Answer 2

Depuis TypeScript 0.9, le lib.d.ts Le fichier utilise des signatures de surcharge spécialisées qui renvoient les types corrects pour les appels à getElementsByTagName.

Cela signifie que vous n'avez plus besoin d'utiliser des assertions de type pour modifier le type :

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

Answer 3

Vous pouvez toujours pirater le système de type Hack utilisant:

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

Answer 4

Ne tapez pas de coulée.Jamais.Utilisez les gardes de type:

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.

Laissez le compilateur faire le travail pour vous et obtenir des erreurs lorsque vos hypothèses se trompent.

Il peut sembler surcharger dans ce cas, mais cela vous aidera beaucoup si vous revenez plus tard et changera le sélecteur, comme l'ajout d'une classe qui manque dans le DOM, par exemple.

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];