What steps can be taken to resolve the error message "Using 'null' as an index type is not allowed."?

In my TypeScript code, I have a variable called lang declared as a string type value, and a variable called direction declared as an object with two elements. I also have a function that is supposed to return the value of the direction object based on the value of lang. However, I am encountering an error when trying to access direction[lang], which says "null' cannot be used as an index type." I am certain that the lang will never be null. Can anyone help me fix this issue?

export const lang = localStorage.getItem('lang') ? localStorage.getItem('lang') : "en";

export const direction  = {
    ru: "rtl",
    en: "ltr"
}

function getDirection() {
    return direction[lang];
}
typescript

Answer №1

The reason for this behavior is due to the defined type of constant lang being string|null. The compiler interprets that if localStorage.getItem('lang') returns null, then the first if-branch will be executed. Since the return type of localStorage.getItem is also null, the variable lang can potentially be null as well. If the second if-branch is executed, then lang could be a string, resulting in its type being string | null. The compiler lacks the capability to deduce that getItem is consistently called with the same argument and therefore fails to infer that the data type of lang should be strictly a string.

Despite this limitation, the following code snippet should function correctly:

export const lang = localStorage.getItem('lang') ?? "en";
export const direction:any  = {
    ru: "rtl",
    en: "ltr"
}

export function getDirection() {
    return direction[lang];
}

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