test.js

const inventory = [
  { name: 'sword', weight: 3},
  { name: 'pickaxe', weight: 2},
   { name: 'bow', weight: 1},
]
var inventoryWeight = 0; 

function addToInventory(item) {
  // if weight of items in inventory
  // less than 10
  inventoryWeight = 0;

  for (let i = 0; i < inventory.length; i++) {
    inventoryWeight += inventory[i].weight;
  }
  
  if (inventoryWeight + item.weight > 10) {
    console.log('Мені важко!')
    return;
  }
  
  inventory.push(item)  

} 

console.log('weight', inventoryWeight, inventory);
addToInventory({ name: 'axe', weight: 1000 })
console.log('weight', inventoryWeight, inventory);
addToInventory({ name: 'knife', weight: 1 })
console.log('weight', inventoryWeight, inventory);