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JavaScript: Check if a given argument is a number

JavaScript fundamental (ES6 Syntax): Exercise-195 with Solution

Write a JavaScript program to check if a given argument is a number.

  • Use typeof to check if a value is classified as a number primitive.
  • To safeguard against NaN, check if val === val (as NaN has a typeof equal to number and is the only value not equal to itself).

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const isNumber = val => typeof val === 'number';

console.log(isNumber('1'));
console.log(isNumber(1));

Sample Output:

false
true

Pictorial Presentation:

JavaScript Fundamental: Check if a given argument is a number.
JavaScript Fundamental: Check if a given argument is a number.

Flowchart:

flowchart: Check if a given argument is a number

Live Demo:

See the Pen javascript-basic-exercise-195-1 by w3resource (@w3resource) on CodePen.


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Previous: Write a JavaScript program to get a boolean determining if the passed value is an object or not.
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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