javascript - From arrow function, push object to array - Stack Overflow

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The async/await function search returns an object similar to:

[
  {
    "title":"page title",
    "url":"/page/url/index.html",
    "content":"blah blah blah"
  },
  ...
]

I need to push each object from val into the array, but when I push using:

results = [];
search(input).then(val=> results.push(val));

I get a nested array similar to (I want to the object entries to be top level in the array):

[
  [
    {title:"...", url:"...", content:"..."},
    {title:"...", url:"...", content:"..."},
  ]
]

So I tried a for in the arrow to push each entry in the object into the array, but that didn't seem to work.

results = [];
search(input).then(val=> for(i in val) {results.push(val[i])});

The async/await function search returns an object similar to:

[
  {
    "title":"page title",
    "url":"/page/url/index.html",
    "content":"blah blah blah"
  },
  ...
]

I need to push each object from val into the array, but when I push using:

results = [];
search(input).then(val=> results.push(val));

I get a nested array similar to (I want to the object entries to be top level in the array):

[
  [
    {title:"...", url:"...", content:"..."},
    {title:"...", url:"...", content:"..."},
  ]
]

So I tried a for in the arrow to push each entry in the object into the array, but that didn't seem to work.

results = [];
search(input).then(val=> for(i in val) {results.push(val[i])});

• javascript
• arrays
• loops
• object
• arrow-functions

Share Improve this question Follow asked Aug 19, 2020 at 14:23 JasonJason 7,6901515 gold badges7979 silver badges129129 bronze badges 3

• 4 Try this: search(input).then(val=> results.push(...val)); – Patrick Hund Commented Aug 19, 2020 at 14:26
• 1 Can't you just use results = val? although I don't really see a purpose of doing this as you'll only be able to access the populated version of results once your promise has resolved, such as in the .then() callback, so you might as well just use val – Nick Parsons Commented Aug 19, 2020 at 14:27
• Try search(input).then(val=> [].concat(results, val)); – Kunal Mukherjee Commented Aug 19, 2020 at 14:33

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 3

Instead of .push(), use .concat()

results = [];
search(input).then(val=> { results = results.concat(val) });

Use spread syntax ...

No For loop needed either.

search(input).then(val=> results = [...results,...val]);

or as @Patrick_Hund suggested

search(input).then(val=> results.push(...val));

here problem is that you are pushing array in to an array.

You need to push the elements of array.

Spreading syntax should fix the issue.

Hope you have clear understanding now.

If you have an array and call a function that will return Promise<array> you don't need to perform any iteration on the result. Concatenate the arrays:

async function getNewData() {
  return Promise.resolve([{
    d: 1
  }, {
    e: 3
  }]);
}

let results = [{
  a: 3
}, {
  b: 4
}];

getNewData().then(newdata => {
  results = results.concat(newdata); 
  // same as results = [...results, ...newdata];

  console.log(results);
});

On the other hand, you could also push using the spread operator

async function getNewData() {
  return Promise.resolve([{
    d: 1
  }, {
    e: 3
  }]);
}

const results = [{
  a: 3
}, {
  b: 4
}];

getNewData().then(newdata => {
  results.push(...newdata); 
  // same as results.push(newdata[0], newdata[1], etc)

  console.log(results);
});

This approach might be confusing as people tend to ignore the variadic nature of Array.prototype.push, but it avoids redeclaring results, so you may declare the target array as const.

If you want to use a for loop, you'll have to do

for(var i=0; i < val.length; i++) {
    results.push(val[i])
}

There are more succinct ways to do this (see Patrick Hund's ment), but that's how you do it with a for loop.

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