Characters are stored on 16 bits
When you see that 256*2**20 characters are in a string, that does not mean that 256 megabytes of memory is allocated. JavaScript stores every character on two bytes (as it is utf16 encoded by the specification).
A word about ropes
Todays browsers (even IE) store strings in an advanced way, most often using a rope datastructure.
- Ropes do not need a coherent memory region to be allocated
- Can even deduplicate substrings, that means
s+sdoes not necessarily use twice the memory ass - Concatenation is very fast
- Element access is a bit slower
By examining some runs in IE and Chrome, I would say that both of them use some lazy evaluation for strings, and will try to expand them occasionally. After running the following snippet, none of the browsers used more memory than before. But if I tried to manipulate the stored window.LONGEST_STRING in the console, IE throw an out of memory error, and chrome froze for a short time, and consumed a lot of memory (>2 GB).
ps: On my laptop IE11 had a maximum string size of 4 GB, Chrome had 512 MB
Browser behaviour
IE11
Chrome47
A faster algorithm to determine max string size
var real_console_log = console.log;
console.log = function(x) {
real_console_log.apply(console, arguments);
var d = document,b=d.body,p=d.createElement('pre');
p.style.margin = "0";
p.appendChild(d.createTextNode(''+x));
b.appendChild(p);
window.scrollTo(0, b.scrollHeight);
};
function alloc(x) {
if (x < 1) return '';
var halfi = Math.floor(x/2);
var half = alloc(halfi);
return 2*halfi < x ? half + half + 'a' : half + half;
}
function test(x) {
try {
return alloc(x);
} catch (e) {
return null;
}
}
function binsearch(predicateGreaterThan, min, max) {
while (max > min) {
var mid = Math.floor((max + min) / 2);
var val = predicateGreaterThan(mid);
if (val) {
min = mid + 1;
} else {
max = mid;
}
}
return max;
}
var maxStrLen = binsearch(test, 10, Math.pow(2, 52)) - 1;
console.log('Max string length is:');
console.log(maxStrLen + ' characters');
console.log(2*maxStrLen + ' bytes');
console.log(2*maxStrLen/1024/1024 + ' megabytes');
console.log('');
console.log('Store longest string');
window.LONGEST_STRING = alloc(maxStrLen);
console.log('Try to read first char');
console.log(window.LONGEST_STRING.charAt(0));
console.log('Try to read last char');
console.log(window.LONGEST_STRING.charAt(maxStrLen - 1));
console.log('Try to read length');
console.log(window.LONGEST_STRING.length);