javascript中charAt方法的替代方法

Question

Here is the task at hand:

Write a function called charAt which accepts a string and an index (number) and returns the character at that index.

The function should return an empty string if the number is greater than the length of the string.

The kicker is that you CAN NOT use the built in charAt method.

Am I doing what it is asking correctly aside from not including the if statement? Also, what would a correct implementation of that look like? (New to JS so my apologies in advance).

function charAt(string, index) {
  var charAt = string[index];
  return charAt;
}

Answer 1

It looks mostly fine, except for one issue - there are a number of odd characters (those composed of surrogate pairs, also sometimes called multibyte characters) which take up more than a single index in a string. An example is 💖. If the string contains a character like this, it will be counted as two indicies in the string:

 function charAt(string, index) { var charAt = string[index]; return charAt; } console.log( charAt('foo💖bar', 3), // Broken character, wrong charAt('foo💖bar', 4), // Broken character, wrong charAt('foo💖bar', 5), // Wrong character (should be "a", not "b") charAt('foo💖bar', 6), // Wrong character (should be "r", not "a") );

If this is a possible problem for your situation, consider using Array.from to turn it into an array first:

 function charAt(string, index) { var charAt = Array.from(string)[index]; return charAt; } console.log( charAt('foo💖bar', 3), charAt('foo💖bar', 4), charAt('foo💖bar', 5), charAt('foo💖bar', 6), );

Or, with the empty string being returned when the index doesn't exist:

 function charAt(string, index) { return Array.from(string)[index] || ''; } console.log( charAt('foo💖bar', 3), charAt('foo💖bar', 4), charAt('foo💖bar', 5), charAt('foo💖bar', 6), ); console.log(charAt('foo💖bar', 123));