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Javascript .reduce example for calculating sum of an array:

arr = [1,2,3,4]

sum = arr.reduce((acc, val) => {
  return acc + val;
})

The same can be written in plain JS as:

arr = [1, 2, 3, 4];
sum = 0;
for(var i = 0; i < arr.length; i++) {
    sum += arr[i];
}

Array.prototype.reduce = function(func,initialValue){
let prev =initialValue
for(let i=0; i< this.length;i++) {
prev = func.apply(this, [prev, this[i]]) + 1
}
return prev
}

let ans = Array.from([1,2,3,4]).reduce((a,b) => a+b, 0)

console.log(ans)

Array.prototype.reduce = function(func,initialValue){
  let prev =initialValue
  for(let i=0; i< this.length;i++) {
      prev = func.apply(this, [prev, this[i]]) + 1
  }
  return prev
}

let ans = Array.from([1,2,3,4]).reduce((a,b) => a+b, 0)

console.log(ans)

Array.prototype.myReduce = function (callback, initValue) {
for(var i=0; i<this.length; i++){
if(initValue !== undefined){
initValue = callback(initValue, this[i], i, this)
}
else {
initValue = this[i]
}
}

return initValue;
}

console.log([1,2,3,4].myReduce((cal, val) => cal + val));
console.log([1,2,3,4].myReduce((cal, val) => cal + val, 3));

Array.prototype.myReduce = function (callback, initValue) {
  for(var i=0; i<this.length; i++){
      if(initValue !== undefined){
          initValue = callback(initValue, this[i], i, this)
      }
      else {
        initValue = this[i]
      }
  }

  return initValue;
}

console.log([1,2,3,4].myReduce((cal, val) => cal + val));
console.log([1,2,3,4].myReduce((cal, val) => cal + val, 3));

Array.prototype.myReduce = function(callback, initial) {
let element = this;
element.map((el)=>{
console.log(initial)
initial = initial !== undefined ? callback(initial,el) : el
})
return initial
}


console.log(arr.myReduce((acc, cu) =>{
return acc + cu
},1))

Array.prototype.myReduce = function(callback, initial) {
let element = this;
element.map((el)=>{
console.log(initial)
initial = initial !== undefined ? callback(initial,el) : el
})
return initial
}


console.log(arr.myReduce((acc, cu) =>{
return acc + cu
},1))

Array.prototype.myReduce = function(callback, initial) {
  let element = this;
  element.map((el)=>{
    console.log(initial)
    initial = initial !== undefined ? callback(initial,el) : el
  })
  return initial
}


console.log(arr.myReduce((acc, cu) =>{
  return acc + cu
},1))