Multi tool use

create element at index position if index does not exist in array

I have an array objects that hold an id and a name

const stages = [{
id: 1,
name: ''
}, {
id: 2,
name: ''
}, {
id: 3,
name: ''
}, {
id: 4,
name: ''
}, {
id: 5,
name: ''
}, {
id: 6,
name: ''
}, {
id: 7,
name: ''
}, {
id: 8,
name: ''
}];


Further I have an array that holds numbers.

const indexPositions = [0, 1, 2, 2, 2, 3, 2, 0];


I want to create a third array that holds arrays. Each number in distances represents the index of the current array within the array.

distances

If the current array does not exist yet I want to create it first. Obviously I have to create new arrays until I get to this index position.

Example:
My array is empty at start. The first index position is 0 so I have to create a new array for this. The next index position is 3 so I have to create more arrays until I have 4 arrays.

All I want to do is to push the stage to its correct level index position. The result of this example would be

const levels = [
[stage1, stage8],
[stage2],
[stage3, stage4, stage5, stage7],
[stage6]
];


Currently my code looks this

$(document).ready(() => {
const levels = ; // the array containing the arrays

stages.forEach((stage, stageIndex) => {
const indexPosition = indexPositions[stageIndex];

const positionDifference = indexPosition - levels.length;

if (positionDifference > 0) {
for (let i = 0; i < positionDifference; i++) { // fill up with empty arrays
levels.push();
}
}

levels[indexPosition].push(stage);
});
});


I get this error Uncaught TypeError: Cannot read property 'push' of undefined and this happens because the indexPosition is out of bounds. If the positionDifference is 0 no array gets created but in the beginning the array is empty.

Uncaught TypeError: Cannot read property 'push' of undefined

positionDifference

I tried setting levels.length to -1 if it is 0 but I still get the error if the difference is 1, I create one array at position 0 and want to access position 1.

levels.length

How can I create an empty array if it does not exist?

3 Answers
3

While I do not fully understand what you want to do, checking existence of an array element is simple, one way of doing that is coercing it to boolean:

const thing=;
function addElem(where,what){
if(!thing[where]) // <- here
thing[where]=;
thing[where].push(what);
}

addElem(2,1);
addElem(2,2);
addElem(2,3);
addElem(5,1);
console.log(thing);

(The indices are deliberately non-continuous, because that does not matter: JavaScript arrays are sparse)

You could use a single loop and add an array for the index if not exists. Then push the wanted value.

var stages = [{ id: 1, name: '' }, { id: 2, name: '' }, { id: 3, name: '' }, { id: 4, name: '' }, { id: 5, name: '' }, { id: 6, name: '' }, { id: 7, name: '' }, { id: 8, name: '' }],
indexPositions = [0, 1, 2, 2, 2, 3, 2, 0],
result = stages.reduce((r, o, i) => {
var index = indexPositions[i];
r[index] = r[index] || ; // take default value for falsy value
r[index].push('stage' + o.id); // instead of string take object
return r;
}, );

console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

You actually were very close! You have a very small issue in your code.

$(document).ready(() => {
const levels = ; // the array containing the arrays

stages.forEach((stage, stageIndex) => {
const indexPosition = indexPositions[stageIndex];

const positionDifference = indexPosition - levels.length + 1; //YOU DID NOT ADD 1 HERE

if (positionDifference > 0) {
for (let i = 0; i < positionDifference; i++) { // fill up with empty arrays
levels.push();
}
}

levels[indexPosition].push(stage);
});
});


When you were calculating the positionDifference, you did not add 1 causing the problem when indexPosition equaled 0 and the for loop did not run and no new arrays were pushed. Just adding one fixed the problem :-)

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