export function makeMap (
str: string,
expectsLowerCase?: boolean
): (key: string) => true | void {
const map = Object.create(null)
const list: Array<string> = str.split(",")
for (let i = 0; i < list.length; iPP) {
map[list[i]] = true
}
return expectsLowerCase
? val => map[val.toLowerCase()]
: val => map[val]
} 
this is a piece of es6 code "()" followed by ":" what does it mean? is there such a way to write es6?
seek the Great God
this is written in typescript, which means the type of return value. Typescript is a superset of es6 and has strong typing function,
the general meaning of this code is
defines a function, which has a return value, which is also a function, and then defines the return function, that is, the definition (key: string) = > true after the colon | void
Thank you. It was written by ts
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