The basic solution is:

function capitalizeFirstLetter(string) {
  return string.charAt(0).toUpperCase() + string.slice(1);
}

console.log(capitalizeFirstLetter('foo')); // Foo

Some other answers modify String.prototype (this answer used to as well), but I would advise against this now due to maintainability (hard to find out where the function is being added to the prototype and could cause conflicts if other code uses the same name / a browser adds a native function with that same name in future).

...and then, there is so much more to this question when you consider internationalisation, as this astonishingly good answer (buried below) shows.

If you want to work with Unicode code points instead of code units (for example to handle Unicode characters outside of the Basic Multilingual Plane) you can leverage the fact that String#[@iterator] works with code points, and you can use toLocaleUpperCase to get locale-correct uppercasing:

function capitalizeFirstLetter([ first, ...rest ], locale = navigator.language) {
  return [ first.toLocaleUpperCase(locale), ...rest ].join('');
}

console.log(capitalizeFirstLetter('foo')); // Foo
console.log(capitalizeFirstLetter("πΆπ²π‘ŒπΌπ²π‘‰")); // "πŽπ²π‘ŒπΌπ²π‘‰" (correct!)
console.log(capitalizeFirstLetter("italya", 'tr')); // Δ°talya" (correct in Turkish Latin!)

For even more internationalization options, please see the original answer below.