Operator delete is unexpectedly slow

Operator delete is unexpectedly slow!

Look at the benchmark.

Delete is the only true way to remove object’s properties without any leftovers, but it works ~ 100 times slower, compared to its “alternative”, setting object[key] = undefined.

This alternative is not the correct answer to this question! But, if you use it with care, you can dramatically speed up some algorithms. If you are using delete in loops and you have problems with performance, read the verbose explanation.

When should one use delete and when set value to undefined ?

An object may be seen as a set of key-value pairs. What I call a ‘value’ is a primitive or a reference to other object, connected to that ‘key’.

Use delete, when you are passing the result object to the code on which you don’t have control (or when you are not sure about your team or yourself).

It deletes the key from the hashmap.

 var obj = {
     field: 1     
 delete obj.field;

Use setting to undefined, when you care about performance. It can give a serious boost to your code.

The key remains on its place in the hashmap, only the value is replaced with undefined. Understand, that for..in loop will still iterate over that key.

 var obj = {
     field: 1     
 obj.field = undefined;

Using this method, not all ways of determining property existence will work as expected.

However, this code:

object.field === undefined

will behave equivalently for both methods.


To summarize, differences are all about ways of determining the property existence, and about for..in loop.

 console.log('* -> "Takes prototype inheritance into consideration, that means it lookups all over prototype chain too."');

 console.log(obj.field === undefined, 'obj.field === undefined', 'You get "undefined" value when querying for "field" in object-hashmap. *');

 console.log(obj["field"] === undefined, 'obj["field"] === undefined', 'Just another way to query (equivalent). *');

 console.log(typeof obj.field === "undefined", 'typeof obj.field === "undefined"', 'Get the value attached to "field" key, and check it\'s type is "undefined". *');

 console.log(! ("field" in obj), '! ("field" in obj)', 'This statement returns true if "field" key exists in the hashmap. False otherwise. *');

 console.log(obj.hasOwnProperty("field"), 'obj.hasOwnProperty("field")', 'This statement returns true if 'field' key exists in the hashmap. The ONLY way NOT to lookup for property in the prototype chain!');
 //Object.keys().indexOf() is an overkill :)

 var counter = 0,
 for (key in obj) {
 console.assert(counter === 0, 'counter === 0', '"field" is not iterated using "for .. in" loop. *');

Source: http://stackoverflow.com/questions/208105/how-to-remove-a-property-from-a-javascript-object


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