JavaScript: Assigns default values for all properties in an object that are undefined
JavaScript fundamental (ES6 Syntax): Exercise-247 with Solution
Write a JavaScript program that assigns default values for all properties in an object that are undefined.
- Use Object.assign() to create a new empty object and copy the original one to maintain key order.
- Use Array.prototype.reverse() and the spread operator (...) to combine the default values from left to right.
- Finally, use obj again to overwrite properties that originally had a value.
Sample Solution:
JavaScript Code:
//#Source https://bit.ly/2neWfJ2
const defaults = (obj, ...defs) => Object.assign({}, obj, ...defs.reverse(), obj);
console.log(defaults({ a: 1 }, { b: 2 }, { b: 6 }, { a: 3 }));
Sample Output:
{"a":1,"b":2}
Flowchart:

Live Demo:
See the Pen javascript-basic-exercise-247-1 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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