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:

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
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