How to map an array of objects to an array of values by key

Question

I want to get the message by key-word in the json structure. For example, the json structure is:

[{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}

Now I want to get all the message and put them into an arrary. How to implement this in javascript? Thanks

`var res = data.map( v => v.message )` – Pranav C Balan Oct 13 '16 at 05:37 — Pranav C Balan, Oct 13 '16 at 05:37
awesome comment , 37 characters that makes sense – Master Yoda Oct 13 '16 at 05:41 — Master Yoda, Oct 13 '16 at 05:41

score 2 · Answer 1 · answered Oct 13 '16 at 05:39

2

var json = [{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}];
var arr = [];
for(i=0;i<json.length;i++) {
 arr.push(json[i].message);
}
console.log(arr);

answered Oct 13 '16 at 05:39

chirag

1,761
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Adam Azad · Accepted Answer · 2016-10-13T05:37:43.973

1

You can use Array.map() or loop over the array and push the message to the array.

var a = [{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}];

var b = a.map(function(obj){
   return obj.message;
});

console.log(b);

edited Oct 13 '16 at 05:37

answered Oct 13 '16 at 05:35

Adam Azad

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"Not bad code". Why do you need an `i` variable what is unused to? Try arrow function instead/ – Tân Oct 13 '16 at 05:37
2

@HappyCoding, I prefer sticking with a code that works for most browsers. ES6 is very new--I know it's exciting--but you need to either use a compiler like Babel or lots of polyfills. :) OP seems to be new to JavaScript, and still making their first steps. Don't rush into ES6 yet. – Adam Azad Oct 13 '16 at 05:40
@Adam Azad Thanks you so much, Adam! But I am just confusing what does the obj mean here? – Xiufen Xu Oct 13 '16 at 05:45
@XiufenXu, `obj` is the object like`{ _id: 123, message: 'hello', username: '1' }` for instance. It's a mere variable referring to the current object in the array. You can change it with any letter you want, but the shorter, the better. – Adam Azad Oct 13 '16 at 05:47
@Adam Azad Got you! Very clear explanation. Thank you so much – Xiufen Xu Oct 13 '16 at 05:51
@XiufenXu, you're welcomed :) – Adam Azad Oct 13 '16 at 05:51

4dgaurav · Answer 3 · 2016-10-13T05:42:09.070

This answers applies everywhere

var initialArray = [{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}]
var newArray = [];
for(i=0;i<initialArray.length;i++){
  newArray.push(initialArray[i].message)
}
console.log(newArray)

or shorter way to write above code

var initialArray = [{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}],
newArray = [];

for (i = 0; i < initialArray.length; i++) newArray.push(initialArray[i].message);
console.log(newArray);

score 1 · Answer 4 · answered Oct 13 '16 at 05:38

Use Array#map method with ES6 arrow function.

var data=[{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}];

var res = data.map(v => v.message);

console.log(res);

With traditional function instead of arrow function.

var data=[{ _id: 123, message: 'hello', username: '1' }, { _id: 456, message: 'world', username: '2'}];

var res = data.map(function(v){
   return  v.message;
});

console.log(res);

score 1 · Answer 5 · answered Oct 13 '16 at 05:43

1

An alternative is to use JsonPath: http://goessner.net/articles/JsonPath/

var messages = jsonPath(yourArray, "$..message");

In my opinion the easiest ways to find stuff in json.

answered Oct 13 '16 at 05:43

Chrisposure

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How to map an array of objects to an array of values by key

5 Answers5