· javascript · 2 min read
10 primitive Javascript validators
No external tools. No custom code. Just use it.
1. Array.isArray()
One of the most straightforward methods for checking if a value is an array is through the Array.isArray() method. It returns true if the passed value is an array and false otherwise.
Array.isArray([]); // true
Array.isArray({}); // false
2. Number.isFinite()
Use Number.isFinite() to determine whether a value is a finite number. It returns true if the value is a number and finite, otherwise false.
Number.isFinite(42); // true
Number.isFinite(Infinity); // false
3. Number.isInteger()
Number.isInteger() allows you to check if a value is an integer. It returns true for integers, and false for non-integers.
Number.isInteger(5); // true
Number.isInteger(5.5); // false
4. Number.isNaN()
The Number.isNaN() method checks if the passed value is NaN (Not a Number). It’s a more reliable way of checking for NaN compared to the global isNaN() function.
Number.isNaN(NaN); // true
Number.isNaN(42); // false
5. Object.is()
Object.is() compares two values for equality, similar to ===, but with a key difference: it treats NaN as equal to NaN.
Object.is('foo', 'foo'); // true
Object.is(NaN, NaN); // true
6. String.prototype.includes()
The includes() method helps you determine if one string contains another. It returns true if the string exists and false otherwise.
'Hello World'.includes('World'); // true
'Hello World'.includes('Foo'); // false
7. String.prototype.startsWith()
You can check if a string starts with a specific substring using startsWith(). It returns true if the string begins with the given substring.
'JavaScript'.startsWith('Java'); // true
'JavaScript'.startsWith('Script'); // false
8. String.prototype.endsWith()
Similar to startsWith(), the endsWith() method checks if a string ends with a particular substring.
'JavaScript'.endsWith('Script'); // true
'JavaScript'.endsWith('Java'); // false
9. Boolean()
The Boolean() function converts any value to true or false. Truthy values (like 1 or a non-empty string) return true, while falsy values (like 0 or null) return false.
Boolean(1); // true
Boolean(0); // false
10. Symbol.hasInstance
Symbol.hasInstance allows customization of the behavior for the instanceof operator. You can define how objects should be checked as instances of a class or constructor.
class MyClass {}
const instance = new MyClass();
MyClass[Symbol.hasInstance](instance); // true
