Question

How to check if a property in object is defined or undefined with JavaScript

Answer 1

const user = {
   languages: {
      a: 'PHP',
      b: 'javascript'
  },
  id: 83829,
  name: undefined
};
 
if (typeof user.languages.a === 'undefined') {
    console.log('user.language.a: undefined');
} else {
    console.log('user.language.a: defined');
}
 
if (typeof user.name === 'undefined') {
    console.log('user.name: undefined');
}

if (typeof user.age === 'undefined') {
    console.log('user.age: undefined');
}
 
if (typeof user.id === 'undefined') {
    console.log('user.id: undefined');
}
else {
    console.log('user.id: defined');
}
 
 
 
 
 
/*
run:
           
"user.language.a: defined"
"user.name: undefined"
"user.age: undefined"
"user.id: defined"
         
*/

 

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Answer 2

const user = {
  id: 928737
};


if (typeof user.name === 'undefined') {
    console.log('undefined'); 
}
else {
    console.log('defined'); 
}
 
  
/*
run:
  
undefined

*/

 

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