Convert the greatest binary number (by number of binary digits) to decimal

Question

Given a number of characters, I want to generate the greatest integer whose binary representation fits in that number of characters.

My code working perfectly fine:

const getValueOf=(y)=>{return parseInt('1'.repeat(y),2)};

/*

| Chars   | Binary        | Decimal
| 4       | 1111          | 15

*/

console.log(getValueOf(4)); // = 15

/*

| Chars   | Binary        | Decimal
| 10      | 1111111111    | 1023

*/

console.log(getValueOf(10)); // = 1023

/* -------------- */

console.time('Time spent on 10000000 executions');
for(let z=0;z<10000000;++z){getValueOf(10)};
console.timeEnd('Time spent on 10000000 executions')

However, this is too slow for my purposes. For 10000000 executions it takes about 666ms. I guess the conversion to text makes it sluggish. How can I make this run faster?

Answer 1

That is a matter of using bit operators:

const getValueOf = y => (1<<y)-1;

So for instance, if y is 4, this shifts a 1 bit four times to the left, which in binary is 10000, and then it subtracts 1, which is 1111 in binary.