Javascript 困惑如何使用 .map 或 .reduce 或 foreach

const users=[
  {firstName:"john",lastName:"Biden",age:26},
  {firstName:"jimmy",lastName:"cob",age:75},
  {firstName:"sam",lastName:"lewis",age:50},
  {firstName:"Ronald",lastName:"Mathew",age:26},  
];

// forEach needs the total initialised

let total=0
users.forEach(user => total+=user.age)
console.log(total)

// map plus reduce (to sum) does not need to initialise

total=users.map(({age}) => age)
.reduce((a,b) => a+b)
console.log(total)

// filter plus reduce also does not need initialising the total

let total50Plus=users.filter(({age}) => age>=50)
.reduce((a,b) => a.age+b.age)   // does not need an accumulator initialised
console.log(total50Plus)

// the standard reduce initialises the accumulator at the end of the function

total=users.reduce((acc,{age}) => acc+=age,0);  
console.log(total)

// full version 

total=users.reduce((accumulator, current) => {
  accumulator += current.age;
  return accumulator;
},0); // initialise the accumulator 
console.log(total)