Printf or String.Format in JavaScript

The printf function and the String.Format() methods are two popular string formatting tools in other programming languages, but JavaScript does not have a built-in equivalent. However, there are several approaches to achieving the same functionality in JavaScript.

Modern way to print string.

let name = "GeeksforGeeks";
let greeting = `Hello, and welcome to ${name}`;
console.log(greeting);

Hello, and welcome to GeeksforGeeks

1. Using console.log() Method

JavaScript provides the console.log() method for printing the value of variables and expressions to the console. This method is often used for debugging and testing purposes, but it can also be used for printing strings and integers in a specific format, similar to the printf() function in other programming languages.

Syntax

console.log() 

const name = 'Geek';
const age = 14;

console.log('My name is ' + name 
    + ' and I am ' + age + ' years old.');

My name is Geek and I am 14 years old.

2. Using Template Literals

In JavaScript, you can use template literals to achieve similar functionality. Template literals allow you to embed expressions inside a string by using ${} syntax.

Syntax

(`${}`)

const name = 'John';
const age = 30;

console.log(`My name is ${name} and I am ${age} years old.`);

My name is John and I am 30 years old.

3. Using the String.format Method

Although JavaScript doesn't have a built-in String.Format method, you can create one yourself using the prototype property of the String object. 

Syntax: To create a custom prototype function in JavaScript, we can define a new method called format on the String prototype. This method will take a string and replace any placeholders in the form of {n} with the corresponding argument passed to the method.

String.prototype.format = function() {
    const args = arguments;
    return this.replace(/{(\d+)}/g, function(match, number) { 
        return typeof args[number] != 'undefined'
            ? args[number]
            : match;
     });
};

String.prototype.format = function () {
    const args = arguments;
    return this.replace(/{(\d+)}/g, function (match, number) {
        return typeof args[number] != 'undefined'
            ? args[number]
            : match;
    });
};

const str = 'Hello {0}, welcome to {1}.';
const name = 'John';
const city = 'New York';
const formattedStr = str.format(name, city);
console.log(formattedStr);

Hello John, welcome to New York.

4. Using the replace() Method

You can use the replace method to replace placeholders in a string with variable values.

Syntax

str.replace(searchValue, replaceValue)

const str = 'Hello {0}, welcome to {1}.';
const name = 'John';
const city = 'India';

const formattedStr = str.replace('{0}', name).replace('{1}', city);
    
console.log(formattedStr);

Hello John, welcome to India.