Hey there! Let's break this down clearly so you can wrap your head around both how every() works and what's happening in your specific code scenario.

First: How does every() decide which elements to compare? #

The Array.prototype.every() method is designed to check every single element in the array it's called on, one by one. Here's the step-by-step process:

• It starts with the first element of the array.
• It passes that element into the callback function you provide (the item => ... part in your code).
• If the callback returns true, it moves on to the next element and repeats the process.
• If the callback ever returns false, it stops immediately (this is called "short-circuit evaluation") and returns false as the final result.
• Only if every element in the array makes the callback return true will every() return true overall.

In short: every() automatically iterates over every element in the original array—you don't have to manually specify which elements to check; it handles that traversal for you.

Second: Breaking down your inventory check code #

Let's walk through your code line by line to clarify what item[0] and item[1] mean:

Your data structures first: #

const order = [['sunglasses', 1], ['bags', 2]];
const inventory = { sunglasses: 1900, pants: 1088, bags: 1344 };

• order is a 2-dimensional array: each element inside it is another array representing a single order line. Each subarray has two values:
  1. The first value is the product name (a string, like 'sunglasses').
  2. The second value is the quantity needed (a number, like 1).

How every() works here: #

When you run order.every(item => inventory[item[0]] >= item[1]):

1. On the first iteration, item is set to the first subarray in order: ['sunglasses', 1]

   • item[0] is the first element of this subarray: 'sunglasses'
   • inventory[item[0]] grabs the value from the inventory object for the key 'sunglasses', which is 1900
   • item[1] is the second element of the subarray: 1
   • The condition checks if 1900 >= 1 → this is true, so every() moves to the next element.

2. On the second iteration, item is set to the second subarray: ['bags', 2]

   • item[0] is 'bags'
   • inventory[item[0]] is 1344
   • item[1] is 2
   • The condition checks if 1344 >= 2 → this is also true.

Since both elements passed the condition, every() returns true overall, meaning all items in the order are in stock.

If you added an order line like ['pants', 1089] to order, here's what would happen:

• The third iteration would check inventory['pants'] (which is 1088) against 1089 → 1088 >= 1089 is false.
• every() would immediately stop iterating and return false, since one item failed the condition.

内容的提问来源于stack exchange，提问作者Cassio Fortunato