string.charAt(x) or string[x]?

Question

Is there any reason I should use string.charAt(x) instead of the bracket notation string[x]?

Answer 1

There is a difference when you try to access an index which is out of bounds or not an integer.

string[x] returns the character at the xth position in string if x is an integer between 0 and string.length-1, and returns undefined otherwise.

string.charAt(x) converts x to an integer using the process explained here (which basically rounds x down if x is a non-integer number and returns 0 if parseInt(x) is NaN) and then returns the character at the that position if the integer is between 0 and string.length-1, and returns an empty string otherwise.

Here are some examples:

"Hello"[313]    //undefined
"Hello".charAt(313)    //"", 313 is out of bounds

"Hello"[3.14]    //undefined
"Hello".charAt(3.14)    //'l', rounds 3.14 down to 3

"Hello"[true]    //undefined
"Hello".charAt(true)    //'e', converts true to the integer 1

"Hello"["World"]    //undefined
"Hello".charAt("World")    //'H', "World" evaluates to NaN, which gets converted to 0

"Hello"[Infinity]    //undefined
"Hello".charAt(Infinity)    //"", Infinity is out of bounds

Another difference is that assigning to string[x] does nothing (which can be confusing) and assigning to string.charAt(x) is an error (as expected):

var str = "Hello";
str[0] = 'Y';
console.log(str);    //Still "Hello", the above assignment did nothing
str.charAt(0) = 'Y';    //Error, invalid left-hand side in assignment

The reason why assigning to string[x] doesn't work is because Javascript strings are immutable.