Quick JavaScript/Typescript debounce function.

debounce.js

const debounce = (f, delay) => {
 
  let timer = null;

  return a=> {

      if(!timer) {
        timer = setTimeout(a=>f(a), delay);
      }else{
        clearTimeout(timer);
        timer = setTimeout(a=>f(a), delay); 
      }
  }
 

}

debounce.ts

const debounce = <A>(f:(a:A)=>void, delay:number) => {
 
  let timer:number = null;

  return (a:A)=> {

      if(!timer) {
        timer = setTimeout(()=>f(a), delay);
      }else{
        clearTimeout(timer);
        timer = setTimeout(()=>f(a), delay); 
      }
  }
 
}

You can't do let timer:number = null;