CODEWITHSUNDEEP
javascript
Arick Conley
Arick Conley

Reputation: 21

Difference between if(prop in obj) and if(obj[prop])?

I'm working through TestDome.com and ran into this question.

Implement the removeProperty function which takes an object and property name, and does the following:

If the object obj has a property prop, the function removes the property from the object and returns true; in all other cases it returns false.

My solution was this:

function removeProperty(obj, prop) {
  if (obj[prop]) {
    delete obj[prop];
    return true;
  } else {
    return false;    
  }
}

The test says that this doesn't work and their solution is:

function removeProperty(obj, prop) {
  if (prop in obj) {
    delete obj[prop];
    return true;
  } else {
    return false;
  }
}

For the life of me I can't figure out why my solution is wrong.

Edit: further examples.

const a = { b: 'c'};

if(a['b']) {console.log(true);} //true
if('b' in a) {console.log(true);} //true

Upvotes: 0

Views: 161

Answers (2)

Willem van der Veen
Willem van der Veen

Reputation: 36630

The difference is the following:

obj = {
  prop: 1
};

// retrieves the property from the object
console.log(obj['prop']);    // 1

// checks if prop is in obj object
console.log('prop' in obj);     // true

In the case of an if statement, both will evaluate to true. However, if the value retrieved from obj['prop'] is coerced to false the if block would not run. For example:

obj = {
  prop: 0
};

// 0 convert to false so 
if (obj['prop']) {
  console.log('executed 1');
}

// prop is a property of obj so 
// 'prop' in obj will evaluate to true
if ('prop' in obj) {
  console.log('executed 2');
}

Upvotes: 2

leogoesger
leogoesger

Reputation: 3830

let's say you have an object 

const a = {apple: 0}

In this case, a['apple'] would be falsy, where as "apple" in a would be truthy.

Therefore, if you want to delete some key, just delete the key directly. I believe you do not even need to check for it before deleting it.

Upvotes: 0

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