How to capitalize the first letter of a string in JavaScript

No Time to Read - Copy/Paste - ES6 Solution

const capitalize = ([firstLetter, ...restOfWord]) =>
  firstLetter.toUpperCase() + restOfWord.join("");

No Time to Read - Copy/Paste - ES5 Solution

function capitalize(string) {
  return string[0].toUpperCase() + string.slice(1);
}

ES6 Explanation

The first thing we want to do is take the string we're trying to capitalize and separate it into two pieces, the first letter, and everything else. To accomplish this we'll use an ES6 feature called Array Destructuring (which conveniently works on strings as well).

If you're not familiar with Array Destructuring, check out Object and Array Destructuring in JavaScript

Just as if we were to destructure the string in the body of the function, we can also do it in the function's parameter.

const capitalize = ([firstLetter, ...restOfWord]) => {};

At this point we have two variables local to our capitalize function - firstLetter and restOfWord. firstLetter is, well, the first letter of the string argument that was passed into capitalize. restOfWord is an array that contains all of the other characters of the argument.

const capitalize = ([firstLetter, ...restOfWord]) => {
  console.log(firstLetter); // 'h'
  console.log(restOfWord); // ["e", "l", "l", "o"]
};

Now that we've separated our string into the first letter and everything else, the next step is to capitalize the first letter. Lucky for us, all JavaScript strings have access to a .toUpperCase method which we can use.

const capitalize = ([firstLetter, ...restOfWord]) => {
  const capitalizedFirstLetter = firstLetter.toUpperCase();
};

The final step is to take the capitalized first letter, combine it with the rest of the word, and return the result.

Remember at this point we have two variables, capitalizedFirstLetter which is a string and restOfWord which is an array. If we convert restOfWord back to a string then we can just return the result of adding capitalizedFirstLetter and the string form of restOfWord back together.

To convert an array into a string, we can use the .join method.

const capitalize = ([firstLetter, ...restOfWord]) => {
  const capitalizedFirstLetter = firstLetter.toUpperCase();
  const restOfWordString = restOfWord.join("");
};

Now that we have the capitalized first letter and the rest of the word as a string, we can add them together and return the result.

const capitalize = ([firstLetter, ...restOfWord]) => {
  const capitalizedFirstLetter = firstLetter.toUpperCase();
  const restOfWordString = restOfWord.join("");
  return capitalizedFirstLetter + restOfWordString;
};

Now to get to our final solution, we can get rid of the variables and use Arrow Function's implicit return.

const capitalize = ([firstLetter, ...restOfWord]) =>
  firstLetter.toUpperCase() + restOfWord.join("");

ES5 Explanation

The ES5 solution follows the same logic as the ES6 solution. We first want to separate the first letter from the rest of the word. In ES5, we can grab the first letter by using bracket notation at the 0 index and we can get the rest of the word by using JavaScript's .slice method.

function capitalize(string) {
  var firstLetter = string[0];
  var restOfWord = string.slice(1); // start at the 1 index
}

Now that we've separated the first letter from the rest of the word, we want to capitalize the first letter then combine it with the rest of the word.

function capitalize(string) {
  var firstLetter = string[0];
  var restOfWord = string.slice(1); // start at the 1 index

  return firstLetter.toUpperCase() + restOfWord;
}

And finally, to get to our final solution we can get rid of the variable and do everything in line.

function capitalize(string) {
  return string[0].toUpperCase() + string.slice(1);
}