Discover how TypeScript's strictNullChecks feature can help you identify null values with ease in your functions

Since Javascript often requires me to check if a value is `!= null && != ''`, I decided to create a function that checks for empty values:

const isEmpty = (variable: any, allowEmptyString?: boolean): boolean => {
    return variable == null || (variable == '' && !allowEmptyString);
};

The downside is that other methods are not aware of what this function does, so I constantly need to use `!` to prevent warnings. For example:

const foo = (val?: number): void => {
    let a = 0;

    if (!isEmpty(val)) {
        a = val;
        // let a: number;
        // Type 'number | undefined' is not assignable to type 'number'.
        // Type 'undefined' is not assignable to type 'number'
    }
};

Currently, my workaround is:

if (!isEmpty(val)) {
    a = val!
}

Is there a way to avoid using `!` to prevent the warnings?

Answer №1

Below is a proposed solution:

function checkEmptyValue(input: string | null | undefined, allowEmptyString?: boolean): input is Exclude<undefined | null, string>
function checkEmptyValue<T>(input: T | null | undefined): input is Exclude<undefined | null, T>
function checkEmptyValue(input: any, allowEmptyString = false): boolean {
    return input === null 
        || input === undefined 
        || (input === '' && !allowEmptyString);
}

Some important notes to consider:

  • Always use === instead of == for strict equality comparison;
  • Handle cases involving undefined values appropriately;
  • By default, an empty string is treated as both undefined and null, unless the allowEmptyString parameter is explicitly set to true;
  • The parameters within the Exclude function are reversed in order to mimic a negation on the boolean value;

Alternatively, consider using an exists function

Another suggestion is to utilize an exists function to simplify the logic without the need for double inversion:

function exists(value: string | null | undefined, allowEmptyString?: boolean): value is string
function exists<T>(value: T): value is NonNullable<T>
function exists(value: any, allowEmptyString = false): boolean {
    return value !== null 
        && value !== undefined 
        && (value !== '' || allowEmptyString);
}

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