Using typescript to iterate through an HTMLCollection with a for loop

Question

Using typescript to iterate through an HTMLCollection with a for loop

Looking to loop through the results from

document.getElementsByTagName("...");

This method returns an HTMLCollection, not an array. Therefore, using forEach directly is not possible. One way to achieve this is as follows:

let elements = document.getElementsByTagName("...");
for (var i = 0, m = elements.length; i < m; i++) {
    let element = elements[i];
}

A similar question exists for javascript: For loop for HTMLCollection elements

Recent updates show that modern browsers now support:

var list = document.getElementsByClassName("events");
for (let item of list) {
    log(item.id);
}

However, there seems to be a typescript compiler error:

error TS2495: Type 'NodeListOf<HTMLParagraphElement>' is not an array type or a string type.

Despite this, the code transpiles to valid Javascript with proper functionality. The compiled output looks like this:

var elements = main_div.getElementsByTagName("p");
for (var _i = 0, elements_1 = elements; _i < elements_1.length; _i++) {
    var element = elements_1[_i];
}

It's reassuring to see that the generated code remains compatible even with older browsers. Yet, it would be ideal to resolve the error message.

The compilerOptions set are as follows:

{
    "compilerOptions": {

        "module": "commonjs",
        "target": "es5",
        "sourceMap": true,
        "rootDir": "src",
        "lib": [
            "es2015",
            "dom"
        ]
    }
}

Attempts were made to modify the lib options in search of a solution, considering that the feature originated in ES6 and underwent revisions. Testing different ecmascript versions did not yield a successful configuration.

typescript

Answer 1

Answer №1

If you'd like to utilize forEach, simply convert it into an array first

const items: Item[] = Array.from(document.querySelectorAll("..."));
items.forEach((item: Item) => {
    // perform a task
})

Answer 2

If you'd like to utilize forEach, simply convert it into an array first

const items: Item[] = Array.from(document.querySelectorAll("..."));
items.forEach((item: Item) => {
    // perform a task
})

Answer 3

Answer №2

To enable support for iterating through elements, the typescript compiler requires the use of the downlevelIteration flag:

{
    "compilerOptions": {
        "target": "es5",
        "downlevelIteration": true
    }
}

Enabling this flag not only resolves any errors but also affects the output generated by the compiler. For example, consider the following TypeScript code:

function compileTest(){
  let elements = document.getElementsByTagName("p");
  for(let element of elements){
    console.log(element);
  }
}

After compilation, the TypeScript code is transformed into JavaScript as shown below:

function compileTest() {
    var e_1, _a;
    var elements = document.getElementsByTagName("p");
    try {
        for (var elements_1 = __values(elements), elements_1_1 = elements_1.next(); !elements_1_1.done; elements_1_1 = elements_1.next()) {
            var element = elements_1_1.value;
            console.log(element);
        }
    }
    catch (e_1_1) { e_1 = { error: e_1_1 }; }
    finally {
        try {
            if (elements_1_1 && !elements_1_1.done && (_a = elements_1.return)) _a.call(elements_1);
        }
        finally { if (e_1) throw e_1.error; }
    }
}

Answer 4

To enable support for iterating through elements, the typescript compiler requires the use of the downlevelIteration flag:

{
    "compilerOptions": {
        "target": "es5",
        "downlevelIteration": true
    }
}

Enabling this flag not only resolves any errors but also affects the output generated by the compiler. For example, consider the following TypeScript code:

function compileTest(){
  let elements = document.getElementsByTagName("p");
  for(let element of elements){
    console.log(element);
  }
}

After compilation, the TypeScript code is transformed into JavaScript as shown below:

function compileTest() {
    var e_1, _a;
    var elements = document.getElementsByTagName("p");
    try {
        for (var elements_1 = __values(elements), elements_1_1 = elements_1.next(); !elements_1_1.done; elements_1_1 = elements_1.next()) {
            var element = elements_1_1.value;
            console.log(element);
        }
    }
    catch (e_1_1) { e_1 = { error: e_1_1 }; }
    finally {
        try {
            if (elements_1_1 && !elements_1_1.done && (_a = elements_1.return)) _a.call(elements_1);
        }
        finally { if (e_1) throw e_1.error; }
    }
}

Answer 5

Answer №3

Here is an example of how this code can be used:

const formInputs = document.getElementsByTagName("input");
for (let i = 0; i < formInputs.length; i++) {
  const inputField = formInputs.item(i);
}

Answer 6

Here is an example of how this code can be used:

const formInputs = document.getElementsByTagName("input");
for (let i = 0; i < formInputs.length; i++) {
  const inputField = formInputs.item(i);
}

Answer 7

Answer №4

If you want to eliminate the error message, one solution is to assign the list as a type of any. TypeScript distinguishes itself from JavaScript through its strong typing system. Therefore, it is recommended to specify the type for your variables. Below is an example with the any type added:

var list: any = document.getElementsByClassName("events");
for (let item of list) {
    console.log(item.id);
}

This concept is known as "contextual typing". Contextual typing occurs when the type of an expression is inferred based on its context. In the code snippet above, the TypeScript type checker used the type of document.getElementsByClassName("events") to determine the type of the ForLoop function in order to highlight the error. If this function were not contextually typed, the list variable would default to type any and no error would be triggered.

If there is explicit type information within a contextually typed expression, the contextual type takes precedence over it.

Contextual typing is applicable in various scenarios such as function arguments, assignment right hand sides, type assertions, object and array literals, and return statements. The contextual type also serves as a candidate type in the best common type resolution.[From TypeScript Document]

Answer 8

If you want to eliminate the error message, one solution is to assign the list as a type of any. TypeScript distinguishes itself from JavaScript through its strong typing system. Therefore, it is recommended to specify the type for your variables. Below is an example with the any type added:

var list: any = document.getElementsByClassName("events");
for (let item of list) {
    console.log(item.id);
}

This concept is known as "contextual typing". Contextual typing occurs when the type of an expression is inferred based on its context. In the code snippet above, the TypeScript type checker used the type of document.getElementsByClassName("events") to determine the type of the ForLoop function in order to highlight the error. If this function were not contextually typed, the list variable would default to type any and no error would be triggered.

If there is explicit type information within a contextually typed expression, the contextual type takes precedence over it.

Contextual typing is applicable in various scenarios such as function arguments, assignment right hand sides, type assertions, object and array literals, and return statements. The contextual type also serves as a candidate type in the best common type resolution.[From TypeScript Document]

Answer 9

Answer №5

Although this response may come a bit late, I wanted to share my insights on how to handle array destructuring in an Angular project. In one of my projects, I encountered the need to use array destructuring with HTMLCollectionOf<T>, as demonstrated below:

const [zoomControl] = document.getElementsByClassName('leaflet-control-zoom');

However, I faced an error stating:

'Type 'HTMLCollectionOf' must have a 'Symbol.iterator' method that returns an iterator.ts(2488)'

There were also instances where iterating over collections proved to be beneficial, such as:

const elementsWithClassName = document.getElementsByClassName('class-name');
for(const a of elementsWithClassName) {
  console.log(a)
}

To resolve these issues, I created a global-types-extensions.d.ts file with the following code snippet:

declare global {
  interface HTMLCollectionOf<T extends Element> {
    [Symbol.iterator](): Iterator<T>;
  }
}
export {};

Implementing this solution effectively addressed the error and warning messages I encountered in TypeScript.

Now, I can freely utilize array destructuring and for...of loops with any HTMLCollectionOf. 😎👍.

Answer 10

Although this response may come a bit late, I wanted to share my insights on how to handle array destructuring in an Angular project. In one of my projects, I encountered the need to use array destructuring with HTMLCollectionOf<T>, as demonstrated below:

const [zoomControl] = document.getElementsByClassName('leaflet-control-zoom');

However, I faced an error stating:

'Type 'HTMLCollectionOf' must have a 'Symbol.iterator' method that returns an iterator.ts(2488)'

There were also instances where iterating over collections proved to be beneficial, such as:

const elementsWithClassName = document.getElementsByClassName('class-name');
for(const a of elementsWithClassName) {
  console.log(a)
}

To resolve these issues, I created a global-types-extensions.d.ts file with the following code snippet:

declare global {
  interface HTMLCollectionOf<T extends Element> {
    [Symbol.iterator](): Iterator<T>;
  }
}
export {};

Implementing this solution effectively addressed the error and warning messages I encountered in TypeScript.

Now, I can freely utilize array destructuring and for...of loops with any HTMLCollectionOf. 😎👍.

Using typescript to iterate through an HTMLCollection with a for loop

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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