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JavaScript: Create an object composed of the properties the given function returns falsey for

JavaScript: Exercise-154 with Solution

Write a JavaScript program to create an object composed of the properties the given function returns falsey for. The function is invoked with two arguments: (value, key).

  • Use Object.keys() and Array.prototype.filter() to remove the keys for which fn returns a truthy value.
  • Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.
  • The callback function is invoked with two arguments: (value, key).

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const omitBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => !fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

console.log(omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'));

Sample Output:

{"b":"2"}

Flowchart:

flowchart: Create an object composed of the properties the given function returns falsey for

Live Demo:

See the Pen javascript-basic-exercise-154-1 by w3resource (@w3resource) on CodePen.


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JavaScript: Tips of the Day

Checking if a key exists in a JavaScript object?

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?

var obj = { key: undefined };
obj["key"] !== undefined // false, but the key exists!

You should instead use the in operator:

"key" in obj // true, regardless of the actual value

If you want to check if a key doesn't exist, remember to use parenthesis:

!("key" in obj) // true if "key" doesn't exist in object
!"key" in obj   // ERROR!  Equivalent to "false in obj"

Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:

obj.hasOwnProperty("key") // true

Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined? var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists! You should instead use the in operator: "key" in obj // true, regardless of the actual value If you want to check if a key doesn't exist, remember to use parenthesis: !("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj" Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty: obj.hasOwnProperty("key") // true For performance comparison between the methods that are in, hasOwnProperty and key is undefined.

Ref: https://bit.ly/2CFNp1X