Typescript Error: type 'string' can't be used to index type X

All you need is to define the index signature:

const allTypes: {[key: string]: boolean} = { jpg: true, gif: true, png: true, mp4: true };

Indexable Types

Similarly to how we can use interfaces to describe function types, we can also describe types that we can “index into” like a[10], or ageMap["daniel"]. Indexable types have an index signature that describes the types we can use to index into the object, along with the corresponding return types when indexing. Let’s take an example:

interface StringArray {
  [index: number]: string;
}

let myArray: StringArray;
myArray = ["Bob", "Fred"];

let myStr: string = myArray[0];

Above, we have a StringArray interface that has an index signature. This index signature states that when a StringArray is indexed with a number, it will return a string.