Consider the following predefined object:
const obj = {
ONE: 'ONE',
TWO: 'TWO'
};
If I attempt to assign the constant obj.ONE to a new type like this:
type TOne = obj.ONE
An error message is displayed stating:
Cannot find namespace 'obj'. or
'ONE' refers to a value, but is being used as a type here.If you want to retrieve the value of a property from an object, one common way is by using the typeof operator, for example:
const TOne = typeof obj.ONE;
However, in Typescript, when dealing with object literals, the types of values are automatically widened by default, which can be problematic as shown in this example:
const obj = {
ONE: 'ONE',
TWO: 'TWO'
};
This results in an obj type like this:
{
ONE: string;
TWO: string;
}
The specific type of the string constant 'ONE' is lost. To prevent this widening behavior, you can either declare the object as const, or specify the property type as as const or as 'ONE':
const obj = {
ONE: 'ONE',
TWO: 'TWO'
} as const;
const obj = {
ONE: 'ONE' as 'ONE',
TWO: 'TWO'
};
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