functionName.js

function getFunctionName(fn) {
    if (typeof fn.name === 'string') {
        return fn.name;
    }
    
    var fnString = fn.toString();
    var firstBracket = fnString.indexOf('(');
    
    return fnString.substr(9, (firstBracket - 9));
}

functionName.ts

declare interface Function extends Function {
    name:string
}

function getFunctionName(fn:Function):string {
    if(typeof fn.name === 'string'){
        return fn.name;
    }
    
    let fnString:string = fn.toString();
    let firstBracket:number = fnString.indexOf('(');
    
    return fnString.substr(9, (firstBracket - 9));
}