How to get or access the properties and methods of header HTML element tags without errors in TypeScript?

Published October 4, 2021

To get or access the properties and methods of the header HTML element tags without having errors or red squiggly lines in TypeScript, you have to assert the type for the header HTML element tag using the HTMLElement interface in TypeScript.

The errors or warnings are shown by the TypeScript compiler since the compiler doesn't know more information on the header HTML element tag ahead of time. But we know that we are referencing the header HTML tag and thus we can tell the compiler to use the HTMLElement interface which contains the declarations for properties and methods.

TL;DR

// get reference to the first 'header'
// HTML element tag in the document
// with type assertion using the HTMLElement interface
const headerTag = document.querySelector("header") as HTMLElement;

// no errors or red squiggly lines will be
// shown while accessing properties or methods
// since we have asserted the type
// for the 'header' HTML element tag 😍
const id = headerTag.id;

To understand it better let's say we are selecting the first header HTML element tag using the document.querySelector() method like this,

// get reference to the first 'header'
// HTML element tag in the document
const headerTag = document.querySelector("header");

Now let's try to get the value of a property called id which is used in the header HTML element tag as an attribute.

It can be done like this,

As you can see that the TypeScript compiler is showing a red squiggly line below the headerTag object. If you hover over the headerTag you can see an error saying Object is possibly 'null'. This is because TypeScript is not sure whether the headerTag object will have properties and methods ahead of time.

Now to avoid this error we can assert the type for the header HTML element tag using the HTMLElement interface in TypeScript.

To do Type assertion we can use the as keyword after the document.querySelector("header") method followed by writing the interface name HTMLElement.

It can be done like this,

// get reference to the first 'header'
// HTML element tag in the document
// with type assertion using the HTMLElement interface
const headerTag = document.querySelector("header") as HTMLElement;

// no errors or red squiggly lines will be
// shown while accessing properties or methods
// since we have asserted the type
// for the 'header' HTML element tag 😍
const id = headerTag.id;

Now if hover over the id property in the headerTag object TypeScript shows more information about the property itself which is again cool and increases developer productivity and clarity while coding.

See the above code live in codesandbox.

That's all 😃!

Feel free to share if you found this useful 😃.


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