adma

Next time, please present properly formatted code with curly braces... its very disorientating to follow if .. else statements that do not use curly brackets...

manager.on("newOffer", function(offer) {
var onlyKeys = (offer.itemsToReceive.every(function(item) {
return item.name == "Mann Co. Supply Crate Key";
}));

if (onlyKeys) {
keyAmount = offer.itemsToReceive.length
console.log("Received trade offer containing " + keyAmount + " keys, accepting");
offer.accept(function(err) {
if (err) console.log(err);
});
} else {
console.log("Trade offer contains non key items. Declining");
offer.decline(function(err) {
if (err) console.log(err);
});
}
} 

var keys = offer.itemsToReceive.map { if (item.name == "Mann Co. Supply Crate Key") { return item; } } var keyAmount = keys.length Something like that should work, and is handy if you wanted to send keys ... you could also do something like
var keyAmount = 0; offer.itemsToReceive.forEach(function(item) { if (item.name == "Mann Co. Supply Crate Key") { keyAmount++; } } // do something with keyAmount

If you want to get the market hash names of each item you're giving, you would probably use forEach or a for loop to cycle through the itemsToGive array ...
e.g
manager.on('newOffer', function(offer) { offer.itemsToGive.forEach(function(item) { console.log("Item to give : " + item.market_hash_name); // Items to give: Chroma 2 Case }); }); a for ... of loop (edited from for..in loop)
manager.on('newOffer', function(offer) { for (var item of offer.itemsToGive) { console.log("Items to give : " + item.market_hash_name); // Items to give: Chroma 2 Case } }); a regular for loop
manager.on('newOffer', function(offer) { for (var i = 0; i < offer.itemsToGive.length; i++) { console.log("Items to give #" + i + " : " + offer.itemsToGive[i].market_hash_name); // Items to give #1 : Chroma 2 Case } });Same deal will apply for itemsToReceive ... or any other array you wanted to cycle through
 
read here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Loops_and_iteration