Hướng dẫn dùng comparisonoperator JavaScript

Comparison and Logical operators are used to test for true or false.

Comparison Operators

Comparison operators are used in logical statements to determine equality or difference between variables or values.

Given that x = 5, the table below explains the comparison operators:

How Can it be Used

Comparison operators can be used in conditional statements to compare values and take action depending on the result:

if [age < 18] text = "Too young to buy alcohol";

You will learn more about the use of conditional statements in the next chapter of this tutorial.

Logical Operators

Logical operators are used to determine the logic between variables or values.

Given that x = 6 and y = 3, the table below explains the logical operators:

Conditional [Ternary] Operator

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

Syntax

variablename = [condition] ? value1:value2 

Example

If the variable age is a value below 18, the value of the variable voteable will be "Too young", otherwise the value of voteable will be "Old enough".

Comparing Different Types

Comparing data of different types may give unexpected results.

When comparing a string with a number, JavaScript will convert the string to a number when doing the comparison. An empty string converts to 0. A non-numeric string converts to NaN which is always false.

When comparing two strings, "2" will be greater than "12", because [alphabetically] 1 is less than 2.

To secure a proper result, variables should be converted to the proper type before comparison:

age = Number[age];
if [isNaN[age]] {
  voteable = "Input is not a number";
} else {
  voteable = [age < 18] ? "Too young" : "Old enough";
}

Try it Yourself »

The Nullish Coalescing Operator [??]

The ?? operator returns the first argument if it is null or undefined. Otherwise it returns the second.

Example

let name = null;
let text = "missing";
let result = name ?? text;

Try it Yourself »

The nullish operator is supported in all browsers since March 2020:

The Optional Chaining Operator [?.]

The ?. operator returns undefined if an object is undefined or null [instead of throwing an error].

Example

// Create an object:
const car = {type:"Fiat", model:"500", color:"white"};
// Ask for car name:
document.getElementById["demo"].innerHTML = car?.name;

Try it Yourself »

The optional chaining operator is supported in all browsers since March 2020:

Comparison and Logical operators are used to test for true or false.

Nội dung chính Show

• Comparison Operators
• How Can it be Used
• Logical Operators
• Conditional [Ternary] Operator
• Comparing Different Types
• JavaScript Comparison Operators
• Example 1: Equal to Operator
• Example 2: Not Equal to Operator
• Example 3: Strict Equal to Operator
• Example 4: Strict Not Equal to Operator
• Example 5: Greater than Operator
• Example 6: Greater than or Equal to Operator
• Example 7: Less than Operator
• Example 8: Less than or Equal to Operator
• JavaScript Logical Operators
• Example 9: Logical AND Operator
• Example 10: Logical OR Operator
• Example 11: Logical NOT Operator
• Which is the comparison operator?
• What is comparison operator with example?
• What is === in JavaScript?
• What is a comparison operator in programming?

Comparison Operators

Comparison operators are used in logical statements to determine equality or difference between variables or values.

Given that x = 5, the table below explains the comparison operators:

How Can it be Used

Comparison operators can be used in conditional statements to compare values and take action depending on the result:

if [age < 18] text = "Too young to buy alcohol";

You will learn more about the use of conditional statements in the next chapter of this tutorial.

Logical Operators

Logical operators are used to determine the logic between variables or values.

Given that x = 6 and y = 3, the table below explains the logical operators:

Conditional [Ternary] Operator

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

Syntax

variablename = [condition] ? value1:value2 

Example

If the variable age is a value below 18, the value of the variable voteable will be "Too young", otherwise the value of voteable will be "Old enough".

Comparing Different Types

Comparing data of different types may give unexpected results.

When comparing a string with a number, JavaScript will convert the string to a number when doing the comparison. An empty string converts to 0. A non-numeric string converts to NaN which is always false.

When comparing two strings, "2" will be greater than "12", because [alphabetically] 1 is less than 2.

To secure a proper result, variables should be converted to the proper type before comparison:

age = Number[age];
if [isNaN[age]] {
  voteable = "Input is not a number";
} else {
  voteable = [age < 18] ? "Too young" : "Old enough";
}

Try it Yourself »

JavaScript Comparison Operators

Comparison operators compare two values and give back a boolean value: either true or false. Comparison operators are used in decision making and loops.

const a = 3;

// greater than or equal operator
console.log[a >= 3]; //true

const a = 3, b = 2;

// less than operator
console.log[a < 2]; // false
console.log[b < 3]; // true