如何在 JavaScript 中迭代（键、值）？

Question

I have a dictionary that has the format of

dictionary = {0: {object}, 1:{object}, 2:{object}}

How can I iterate through this dictionary by doing something like

for ((key, value) in dictionary) {
    //Do stuff where key would be 0 and value would be the object
}

Answer 1

tl;dr

1. In ECMAScript 2017, just call Object.entries(yourObj) .
2. In ECMAScript 2015, it is possible with Map s.
3. In ECMAScript 5, it is not possible.

ECMAScript 2017

ECMAScript 2017 introduced a new Object.entries function. You can use this to iterate the object as you wanted.

'use strict';

const object = {'a': 1, 'b': 2, 'c' : 3};

for (const [key, value] of Object.entries(object)) {
  console.log(key, value);
}

Output

a 1
b 2
c 3

---

ECMAScript 2015

In ECMAScript 2015, there is not Object.entries but you can use Map objects instead and iterate over them with Map.prototype.entries . Quoting the example from that page,

var myMap = new Map();
myMap.set("0", "foo");
myMap.set(1, "bar");
myMap.set({}, "baz");

var mapIter = myMap.entries();

console.log(mapIter.next().value); // ["0", "foo"]
console.log(mapIter.next().value); // [1, "bar"]
console.log(mapIter.next().value); // [Object, "baz"]

Or iterate with for..of , like this

'use strict';

var myMap = new Map();
myMap.set("0", "foo");
myMap.set(1, "bar");
myMap.set({}, "baz");

for (const entry of myMap.entries()) {
  console.log(entry);
}

Output

[ '0', 'foo' ]
[ 1, 'bar' ]
[ {}, 'baz' ]

Or

for (const [key, value] of myMap.entries()) {
  console.log(key, value);
}

Output

0 foo
1 bar
{} baz

---

ECMAScript 5:

No, it's not possible with objects.

You should either iterate with for..in , or Object.keys , like this

for (var key in dictionary) {
    // check if the property/key is defined in the object itself, not in parent
    if (dictionary.hasOwnProperty(key)) {           
        console.log(key, dictionary[key]);
    }
}

Note: The if condition above is necessary only if you want to iterate over the properties which are the dictionary object's very own. Because for..in will iterate through all the inherited enumerable properties.

Or

Object.keys(dictionary).forEach(function(key) {
    console.log(key, dictionary[key]);
});

Answer 2

Try this:

dict = {0:{1:'a'}, 1:{2:'b'}, 2:{3:'c'}}
for (var key in dict){
  console.log( key, dict[key] );
}

0 Object { 1="a"}
1 Object { 2="b"}
2 Object { 3="c"}

Answer 3

The Object.entries() method has been specified in ES2017 (and is supported in all modern browsers ):

for (const [ key, value ] of Object.entries(dictionary)) {
    // do something with `key` and `value`
}

Explanation:

• Object.entries() takes an object like { a: 1, b: 2, c: 3 } and turns it into an array of key-value pairs: [ [ 'a', 1 ], [ 'b', 2 ], [ 'c', 3 ] ] .

• With for ... of we can loop over the entries of the so created array.

• Since we are guaranteed that each of the so iterated array items is itself a two-entry array, we can use destructuring to directly assign variables key and value to its first and second item.

Answer 4

WELCOME TO 2020 *Drools in ES6*

Theres some pretty old answers in here - take advantage of destructuring. In my opinion this is without a doubt the nicest (very readable) way to iterate an object.

 const myObject = { nick: 'cage', phil: 'murray', }; Object.entries(myObject).forEach(([k,v]) => { console.log("The key: ", k) console.log("The value: ", v) })

Edit:

As mentioned by Lazerbeak, map allows you to cycle an object and use the key and value to make an array.

 const myObject = { nick: 'cage', phil: 'murray', }; const myArray = Object.entries(myObject).map(([k, v]) => { return `The key '${k}' has a value of '${v}'`; }); console.log(myArray);

Edit 2:

To explain what is happening in the line of code:

Object.entries(myObject).forEach(([k,v]) => {}

Object.entries() converts our object to an array of arrays:

[["nick", "cage"], ["phil", "murray"]]

Then we use forEach on the outer array:

1st loop: ["nick", "cage"]
2nd loop: ["phil", "murray"]

Then we "destructure" the value (which we know will always be an array) with ([k,v]) so k becomes the first name and v becomes the last name.

Answer 5

Try this:

var value;
for (var key in dictionary) {
    value = dictionary[key];
    // your code here...
}

Answer 6

You can do something like this :

dictionary = {'ab': {object}, 'cd':{object}, 'ef':{object}}
var keys = Object.keys(dictionary);

for(var i = 0; i < keys.length;i++){
   //keys[i] for key
   //dictionary[keys[i]] for the value
}

Answer 7

I think the fast and easy way is

Object.entries(event).forEach(k => {
    console.log("properties ... ", k[0], k[1]); });

just check the documentation https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries

Answer 8

using swagger-ui.js

you can do this -

_.forEach({ 'a': 1, 'b': 2 }, function(n, key) {
    console.log(n, key);
 });

Answer 9

You can use below script.

var obj={1:"a",2:"b",c:"3"};
for (var x=Object.keys(obj),i=0;i<x.length,key=x[i],value=obj[key];i++){
    console.log(key,value);
}

outputs
1 a
2 b
c 3

Answer 10

As an improvement to the accepted answer, in order to reduce nesting, you could do this instead, provided that the key is not inherited:

for (var key in dictionary) {
    if (!dictionary.hasOwnProperty(key)) {
        continue;
    }
    console.log(key, dictionary[key]);
}

Edit: info about Object.hasOwnProperty here

Answer 11

You can use JavaScript forEach Loop:

myMap.forEach((value, key) => {
    console.log('value: ', value);
    console.log('key: ', key);
});