manipulation with objects in array

Question

This is challenging task for me. I have an array of object. I need to loop thru an array , if there is "message" key, add it to a new array. But there is a tricky thing here. If 'render' is true , I need to get "stageID"(for instance in fisrt element stadeID = 3 ) and move to the third element in that array, get 'message' if there is one , again add it to array and check for "render" status,keep looping until render is false. If in element no 'message' key , but render is true , anyway I need to move to elements by it's stageID.

P/S I always need to get message from first element , doesn't matter if render is true or not

[
 {
 "algID": 0,
 "render": true,
 "message": "first message",
 "stageID": "3"
 },
 {
 "algID": 0,
 "render": false,
 "message": "second message",
 "message_type": "text"
 },
 {
 "algID": 0,
 "render": true,
 "message": "third message",
 "message_type": "text",
 "stageID": "5"
 },
 {
 "algID": 0,
 "render": false
 },
 {
 "algID": 0,
 "render": false,
 "stageID": "4"
 },
 {
 "algID": 0,
 "render": false
 }
]

Here is my function. I'm adding first element 'message' to array , checking for 'render' but I have no idea how loop it moving thru array using 'stageID'

displayData(step) {
 let arrayWithMessage=[]
 for (let z = 0; z < step.length; z++) {
 if ("message" in step[z]) {
 arrayWithMessage.push(step[z].message)
 }
 if (step[z].render === true) {
 console.log("reder true", step[step[z].stageID - 1])
 }
 }
 }

expected output: ['first message', 'third message']

• adiga: Please post the expected output
• Yerlan Yeszhanov: @adiga posted below code
• Nina Scholz: what means "move to the third element in that array"?, what happens to "second message"?

Answer 1

You could iterate the array and take either the next element or the stageID - 1 as new index.

var data = [{ algID: 0, render: true, message: "first message", stageID: "3" }, { algID: 0, render: false, message: "second message", message_type: "text" }, { algID: 0, render: true, message: "third message", message_type: "text", stageID: "5" }, { algID: 0, render: false }, { algID: 0, render: false, stageID: "4" }, { algID: 0, render: false }],
 result = [],
 i = 0;
while (data[i]) {
 if (data[i].message) result.push(data[i].message);
 if (data[i].render) {
 i = data[i].stageID - 1;
 continue;
 }
 i++;
}
console.log(result);

Answer 2

Let your array is stored in the variable a

var messages = [];
function getMessage(index) {
 if (a[index - 1].message !== undefined) {
 messages.push(a[index - 1].message);
 }
 if (a[index - 1].render) {
 getMessage(a[index - 1].stageID);
 }
}
getMessage(1);

Answer 3

Edit: is stageID - 1 the index of the next linked post? If yes, this solution or Ninas is corrent.

I interpreted I need to get "stageID"(for instance in fisrt element stadeID = 3 ) and move to the third element in that array as, if stageID = 3, the next element is the 3rd element of the array.

---

If I understood the logic correctly, something like this could work:

We create a seperate function that will extract all the posts where render is true and a message exists.

Then we can just map that array to extract the posts.

The advantage being that we can easily use it in a mapping function to do that for all the posts.

const posts = [
 { "algID": 0, "render": true, "message": "first message", "stageID": "3" },
 { "algID": 0, "render": false, "message": "second message", "message_type": "text" },
 { "algID": 0, "render": true, "message": "third message", "message_type": "text", "stageID": "5" },
 { "algID": 0, "render": false },
 { "algID": 0, "render": false, "stageID": "4" },
 { "algID": 0, "render": false }
];
const find_message_chain = post => {
 const index = parseInt( post.stageID ) - 1;
 // If we have render: true, we will need to look for the next post as well.
 // By recursively calling find_chain and concatenating all the results, we get a single, one lvel deep array instead of nested arrays we have to .flat() at the end.
 if ( post.render && post.message ) return [ post.message, ...find_message_chain( posts[ index ]) ];
 // If we do not have render true, but do have a message, this post will br the last one in the chain, so we return it.
 else if ( post.message ) return [ post.message ];
 // If no message exists, we do not want to return the post, since the chain ends.
 else return [];
};
const messages_for_post_0 = find_message_chain( posts[ 0 ] );
console.log( messages_for_post_0 );
const all_messages_per_post = posts.map( find_message_chain );
console.log( all_messages_per_post );

Sidenote 1: I would add an actual ID to the posts, so we do not have to rely on array index of the source data ( posts ) to determine which post is linked to which other post.

Sidenote 2: The recursive chain function can easily be made less exact to allow for arbitary extraction of any value, not only messages.

• Yerlan Yeszhanov: is stageID - 1 the index of the next linked post? yes!
• Yerlan Yeszhanov: What if render is true and stageID refers it to the element where render is false?
• Shilly: If the stageID refers to an element who's render is false, then the chain stops with adding the message from that last element where render was false, as per your description: If render is true, add own message and add message of element stageID refers to, if it exists. If render is true on the stageID element, continue.
• Shilly: So if you only have to add elements where both render is true and message exists, the code becomes easier as well. But then your description of the logic is even more multi-interpretable.