In the land of coding, there are two constants: const which sets a value at declaration, and let which allows for variables to be changed. But what about a special Typescript (or javascript) variable that starts as undefined, and once defined, remains forever unchangeable?
Implementing this logic in JavaScript can be a bit tricky when using a single primitive value. However, you can ensure type safety by incorporating setter and getter methods. The setter method checks if the value has already been set, while the getter method retrieves it.
const customObject = (() => {
let value: number;
return {
setValue: (newValue: number) => {
if (value !== undefined) {
throw new Error('Value has already been set');
}
value = newValue;
},
getValue: () => {
if (value === undefined) {
throw new Error('Value has not been set yet');
}
return value;
}
};
})();
// will throw an error
const result1 = customObject.getValue();
// will return 3
customObject.setValue(3);
const result2 = customObject.getValue();
// will throw an error if run just after the above
customObject.setValue(5);
It appears that in this case, const cannot be used and only let is allowed. Additionally, you must specify the initial type to prevent it from inferring to any by default. Alternatively, you can use a union type like string| number so that you can later assign it to either of those types.
let value: string; // string | number
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