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JavaScript: Check whether the provided value is of the specified type

JavaScript fundamental (ES6 Syntax): Exercise-208 with Solution

Write a JavaScript program to check whether the provided value is of the specified type.

  • Ensure the value is not undefined or null using Array.prototype.includes().
  • Compare the constructor property on the value with type to check if the provided value is of the specified type.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const is = (type, val) => ![, null].includes(val) && val.constructor === type;
console.log(is(Array, [1])); // true
console.log(is(ArrayBuffer, new ArrayBuffer()));
console.log(is(Map, new Map()));
console.log(is(RegExp, /./g));
console.log(is(Set, new Set()));
console.log(is(WeakMap, new WeakMap()));
console.log(is(WeakSet, new WeakSet()));
console.log(is(String, ''));
console.log(is(String, new String('')));
console.log(is(Number, 1));
console.log(is(Number, new Number(1)));
console.log(is(Boolean, true));
console.log(is(Boolean, new Boolean(true)));

Sample Output:

true
true
true
true
true
true
true
true
true
true
true
true
true

Flowchart:

flowchart: Check whether the provided value is of the specified type.

Live Demo:

See the Pen javascript-basic-exercise-208-1 by w3resource (@w3resource) on CodePen.


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Next: Write a JavaScript program to get a list of elements that exist in both arrays, using a provided comparator function.

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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