JavaScript參數映射（）函數

Question

請解釋這個JavaScript語言結構：

cursor => cursor.map(doc => doc._key) 

在這種情況下

collection.all().then(
    cursor => cursor.map(doc => doc._key) // this line 
).then(
    keys => console.log('All keys:', keys.join(', ')), 
    err => console.error('Failed to fetch all documents:', err) 
); 

不瞭解doc => doc._key作爲參數傳遞給map()功能。爲什麼它不適用於doc => { key: doc._key, name: doc.name}？

Answer 1

讓我們按行劃分： 也有一些documentation on arrow functions。

collection.all() 

// give me all the documents in the collection 

.then(

    cursor 

// take a cursor, which goes over each item in the collection 

=> 

// you can think of this as "take the cursor as input into an anonymous function, and return..." 

cursor.map(

// a map over the cursors output 

doc => doc._key) 

// each document the cursor finds, return the documents key 

).then(

    keys => console.log('All keys:', keys.join(', ')), 

// take the resulting keys, and console log their value 

    err => console.error('Failed to fetch all documents:', err) 

// if there are any errors, please log those as well. 
);