Tips for sidestepping nested TypeScript if statements by declaring a variable like so: `let x: type | undefined =

Is there a more efficient approach to handling type checking for variables defined as type | type2? For instance, consider the following code snippet:

    if (e) {
        let targetElement: Element | undefined = e.toElement;
        if (targetElement) {
            let medicineCategory: string | null = targetElement.textContent;
            if (medicineCategory) {
                $('#selected-medicine-info').text(medicineCategory);
            }
        }
    }

Having multiple checks in the code for !undefined and !null can seem cumbersome, especially when dealing with deeper nesting.

I came up with a slightly cleaner solution that allows me to easily determine if the value of textContent was null:

    let targetElement: Element | undefined = e.toElement;
    if (targetElement) {
        let medicineCategory: string | null = (targetElement.textContent !== null) ? targetElement.textContent : "null";
        $('#selected-medicine-info').text(medicineCategory);
    }

Answer №1

If you want to handle errors automatically, consider using LoDash. With _.get, errors are taken care of for you. Whether e is null/undefined or the path doesn't exist, a default value will be returned. If no default is provided, undefined is returned.

// When using _.get with an object, path, and default value,
// Typescript infers the type as `string` due to the empty string default
const medicineCategory = _.get(e, ['toElement', 'textContext'], '');

if (medicineCategory) 
   $('#selected-medicine-info').text(medicineCategory);

To learn more, visit: https://lodash.com/docs/4.17.10#get

There's a reason why LoDash is one of npm's most relied upon libraries. (https://www.npmjs.com/ check bottom of page).

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