check if key exist in object javascript

Question

I have object in which if key does not exist set to null using javascript.

If the key doesnot exist in object obj like ccy does not exist cn MY and CA, how to assign and set to null using javascript.

var obj=[{
  "cn": "TH",
  "lang": "thai",
  "bank": "yes",
  "ccy": "THB"
},{
   "cn": "MY",
   "lang": "malay",
},{
   "cn": "CA",
   "lang": "english",
   "bank": "yes"
}]

render(){
  obj.map(e=>
  return html`
    <h1>${e.cn}</h1>
    <h5>${e.lang}</h5>
    <h6>${e.ccy}</h6>
`
 )
}

Answer 1

The easiest thing to do, as long as you can be sure e.ccy won't be falsy (false, undefined, null, 0, "", or NaN) is to use the logical OR operator ||:

${e.ccy || null}

Answer 2

If you don't want to write default fallback values into template and want to keep template clean, you could also preprocess data and set missing values before rendering:

var obj = [{
  "cn": "TH",
  "lang": "thai",
  "bank": "yes",
  "ccy": "THB"
}, {
  "cn": "MY",
  "lang": "malay",
}, {
  "lang": "english",
  "bank": "yes"
}]

obj = obj.map(({
  cn = null,
  ccy = null,
  ...rest
}) => Object.assign(rest, { cn, ccy }))

console.log(obj)

Answer 3

You can use the operator in to check if the key exists as follow:

The operator in is the most secure way for avoiding wrong checks on falsy values.

{"ccy" in e ? e.ccy : null}

Answer 4

Any key not defined in a javascript object will be considered to be undefined so you could either check

(typeof obj.ccy === "undefined") ? null : obj.ccy;
(obj.ccy == undefined) ? null : obj.ccy;

If you are not familiar with ternary expressions. You could also write the above as this.

if(obj.ccy == undefined){
   obj.ccy = null;
}

Answer 5

There are multiple ways to check if a given key exists in an object.

if(!obj[0].hasOwnProperty('cn')) {
    obj[o].cn = null
}

or

if(typeof obj[0].cn === 'undefined') {
    obj[0].cn = null;
}

or

if(obj[0].cn === undefined) {
    obj[0].cn = null;
}

The latter two are very similar in this case, typeof is usually used when you want to check if a variable is defined and accessible in the code.

Now the real example is like so:

var missing = ['cn', 'ccy'];
for(var i = 0; i < obj.length; i++) {
    for(var j = 0; j < missing.length; j++) {
        if(!obj[i].hasOwnProperty(missing[j])) {
            obj[i][missing[j]] = null;
        }
    }
}

Answer 6

var q = [...Object.keys(obj[0]),...Object.keys(obj[1]),...Object.keys(obj[2])]
for(let i=0;i<obj.length;i++){
 for(let k=0;k<q.length;k++){
   if(!(q[k] in obj[i])){
    obj[i][q[k]] = null
  }
 }
}
console.log(obj)