Function Duplicating Array on Output

maybe something like that ?

const Info_Buyer = 
  { Name_Buyer : 'John'
  , Spent_Rest : [50, 100, 300, 10, 80]  
  , Cal_Tip() 
    { // The function stores the amount spent for each restaurant and tips depending on the amount spent
    return this.Spent_Rest.map(x=>
      {
      if (x <= 50)  return x * 0.2
      if (x <= 200) return x * 0.15
      return x * 0.1
      })
    }
  , Cal_Total()
    { // The function sums the value of the tip and the amount spent on the order, showing the total expense as output
    return this.Cal_Tip().reduce((ac,el,n)=>ac +el +this.Spent_Rest[n], 0)
    }
  }

let tip   = Info_Buyer.Cal_Tip()
  , total = Info_Buyer.Cal_Total()

console.log('tip   =', JSON.stringify(tip));
console.log('total =', total );

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Expand snippet

in your code you do somethig like that:

function Cal_Tip() {
    let val
    if (x < 50)             val = x * 0.2   // x = 0...49
    else if(x>50 && < 200)  val = x * 0.15  // x = 51...199
    else                    val = x * 0.1   // x = 50 or x = 200...infinite
    return val
}

and you miss x = 50 !

the else if(x>50 && < 200)
should be else if(x>=50 && < 200)

but it is useless because values from 0...49 are tested before and can't happen in this else if

so your code should be:

function Cal_Tip() {
    let val
    if (x <= 50)      val = x * 0.2  
    else if(x <= 200) val = x * 0.15 
    else             val = x * 0.1 
    return val
}

and my code is

function Cal_Tip() {
  if (x <= 50 )  return = x * 0.2    // if (x < 50 ) the function is finished and return x * 0.2
  if (x <= 200)  return = x * 0.15  // if (x < 200) the function is finished and return x * 0.15
  return = x * 0.1                // in aother cases...
}

javascript offers many methods on array object (on JS everythig is object!)

the doc on mdn on the method array.map (with examples) => https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

the doc on mdn on the method array.reduce => https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
don't forget to use initialValue for this one.

arrows functions https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions
quick example:

function functionA(param) { return param *10 }
// become 
functionA = param => param*10  // the arrow make the return too

function functionB(param1,param2) {
  let sum = param1+param2; 
  return sum/2 
}
// become 
functionB =(param1,param2)=>{ // more than 1 arguments -> you should use parenthesis
  let sum = param1+param2;  // + more than 1 line of -> you should use brackets
  return sum/2 
}

In fact my solution was simpler than I expected. Instead of this.Cal_Tip () I just called the end result of my function (which would be the Tip array), as @Barmar said the function was called 2 times in Cal_Total. When the Cal_Tip function is called on the out-of-object variable declaration, the Tip value is automatically loaded into the this.Tip declaration. Thanks for the answers.

const Info_Buyer = {
  Name_Buyer: 'John',
  Spent_Rest: [50, 100, 300, 10, 80],
  Tip: [],
  Total_Spent: 0,
  // The function stores the amount spent for each restaurant and tips depending on the amount spent
  Cal_Tip: function() {
    let n = 0
    while (n < this.Spent_Rest.length)
    {
      if (this.Spent_Rest[n] <= 50)
      {
        this.Tip.unshift(this.Spent_Rest[n] * 0.2);
      } 
      else if (this.Spent_Rest[n] <= 200)
      {
        this.Tip.unshift(this.Spent_Rest[n] * .15);
      }
      else 
      {
        this.Tip.unshift(this.Spent_Rest[n] * .1);
      }
      n++
    }
    return this.Tip
  },
  // The function sums the value of the tip and the amount spent on the order, showing the total expense as output
  Cal_Total: function() {
    let tip = this.Tip //<------ MODIFIED AREA
    let n = 0
    while (n < this.Spent_Rest.length) {
      this.Total_Spent += this.Spent_Rest[n] + tip[n]
      n++
    }
    return this.Total_Spent
  }
}
total = Info_Buyer.Cal_Tip()
tip = Info_Buyer.Cal_Total()
console.log(total, tip);