Two things: first, Array.find() returns the first matching element, undefined if it finds nothing. Array.filter returns a new array containing all matching elements, [] if it matches nothing.

Second thing, if you want to match 4,5, you have to look into the string instead of making a strict comparison. To make that happen we use indexOf which is returning the position of the matching string, or -1 if it matches nothing.

Example:

const arr = [
  {
    name: 'string 1',
    arrayWithvalue: '1,2',
    other: 'that',
  },
  {
    name: 'string 2',
    arrayWithvalue: '2',
    other: 'that',
  },
  {
    name: 'string 2',
    arrayWithvalue: '2,3',
    other: 'that',
  },
  {
    name: 'string 2',
    arrayWithvalue: '4,5',
    other: 'that',
  },
  {
    name: 'string 2',
    arrayWithvalue: '4',
    other: 'that',
  },
];

const items = arr.filter(item => item.arrayWithvalue.split(',').indexOf('4') !== -1);

console.log(items);

Finding the array element:

let arr = [
    { name:"string 1", value:"this", other: "that" },
    { name:"string 2", value:"this", other: "that" }
];

let obj = arr.find(o => o.name === 'string 1');

console.log(obj);

Replacing the array element:

let arr = [
    { name:"string 1", value:"this", other: "that" },
    { name:"string 2", value:"this", other: "that" }
];

let obj = arr.find((o, i) => {
    if (o.name === 'string 1') {
        arr[i] = { name: 'new string', value: 'this', other: 'that' };
        return true; // stop searching
    }
});

console.log(arr);

First of all you need to rotate your loop of array then you need to rotate your json loop.

const obj = [{
 a:'a',
 b:'b',   
},
{
 c:'c',
 d:'d',   
},
{
 e:'e',
 f:'f',   
}]
let keys = [];
for(let i= 0; i < obj.length; i++){
  for(let key in obj[i]){
    keys.push(key);
  }
}
console.log(keys);

You should invoke the Array.prototype.filter function there.

var filteredArray = YourArray.filter(function( obj ) {
    return obj.value === 1;
});

.filter() requires you to return the desired condition. It will create a new array, based on the filtered results. If you further want to operate on that filtered Array, you could invoke more methods, like in your instance .map()

var filteredArray = YourArray.filter(function( obj ) {
    return obj.value === 1;
}).map(function( obj ) {
    return obj.id;
});

console.log( filteredArrays ); // a list of ids

... and somewhere in the near future, we can eventually use the Arrow functions of ES6, which makes this code even more beauty:

var filteredArray = YourArray.filter( obj => obj.value === 1 ).map( obj => obj.id );