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JavaScript: Compare two objects to determine if the first one contains equivalent property values to the second one, based on a provided function

JavaScript fundamental (ES6 Syntax): Exercise-179 with Solution

Write a JavaScript program to compare two objects to determine if the first one contains equivalent property values to the second one, based on a provided function.

  • Use Object.keys() to get all the keys of the second object.
  • Use Array.prototype.every(), Object.prototype.hasOwnProperty() and the provided function to determine if all keys exist in the first object and have equivalent values.
  • If no function is provided, the values will be compared using the equality operator.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const matchesWith = (obj, source, fn) =>
  Object.keys(source).every(
    key =>
      obj.hasOwnProperty(key) && fn
        ? fn(obj[key], source[key], key, obj, source)
        : obj[key] == source[key]
  );
const isGreeting = val => /^h(?:i|ello)$/.test(val);

console.log(matchesWith(
  { greeting: 'hello' },
  { greeting: 'hi' },
  (oV, sV) => isGreeting(oV) && isGreeting(sV)
));

Sample Output:

true

Flowchart:

flowchart: Compare two objects to determine if the first one contains equivalent property values to the second one, based on a provided function

Live Demo:

See the Pen javascript-basic-exercise-179-1 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