How to Capitalize First Letter of String in JavaScript

In this quick tutorial, we’ll learn how to capitalize the first letter of a String in JavaScript.

‘capitalize’ Function

You can use this custom made capitalize() function to capitalize the first letter of a string:

// es5 way
function capitalize(string) {
  return string.charAt(0).toUpperCase() + string.slice(1);
}

// es6 way using destructuring 
const capitalize = ([first,...rest]) => first.toUpperCase() + rest.join('');

‘capitalize’ Function Details

Let’s look at the steps involved to come up with capitalize() function:

1. Get the first letter of the string using charAt() method

   const string = "string";
   
   string.charAt(0);  // Returns "s"
   

2. Convert the first letter to uppercase using toUpperCase() method

   const string = "string";
   
   string.charAt(0).toUpperCase();  // Returns "S"
   

3. Get the rest of the string except first letter using slice() method

   const string = "string";
   
   string.slice(1);  // Returns "tring"
   

   Note that slice(1) means get a substring from index 1 to end of the string. Alternatively, You can also use substring(1).
4. Finally, add the first uppercase letter to rest of the string

   var string = "string";
   
   function capitalize(string) {
     return string.charAt(0).toUpperCase() + string.slice(1);
   }
   
   capitalize(string);  // Returns "String"
   

Add ‘capitalize’ to String methods

We can also add our custom made capitalize() function to String.prototype methods so that we can directly use that on a string.

var string = "string";

/* this is how methods are defined in prototype of any built-in Object */
Object.defineProperty(String.prototype, 'capitalize', {
    value: function () {
        return this.charAt(0).toUpperCase() + this.slice(1);
    },
    writable: true, // so that one can overwrite it later
    configurable: true // so that it can be deleted later
});

string.capitalize(); // Returns "String"

Capitalize First Letter of each word in a given String

We can use the capitalizeSentence function to capitalize first letter of each word in a sentence:

function capitalizeSentence(sentence) {
    return sentence
      .split(" ")
      .map(string => string.charAt(0).toUpperCase() + string.slice(1))
      .join(" ");
}

capitalizeSentence("a quick brown fox jumps over the lazy dog");
// "A Quick Brown Fox Jumps Over The Lazy Dog"