map_reduce.js

//map function.
var map=(array,operator)=>{

let maparr=[];

for(let i=0;i<array.length;i++){
    maparr.push(operator(array[i],i,array))
}
return maparr;
}

map([1,2,3],(a)=>a*2); //(3) [2, 4, 6]

//reduce function.
var reduce=(...args)=>{
let [array,operator,initialval]=args;
let accumulator=args.length===3?initialval:array[0];

for(let i=3-args.length;i<array.length;i++){  
    accumulator=operator(accumulator,array[i],i,array);
}
return accumulator;
}

reduce([1,2,3],(acc,i)=>acc+i); //6