technical, programming, JavaScript
13:26 -0400

Improved versions of left-pad

Shorter version (removed unneeded variable, and change to for loop: 9 lines of code instead of 11):

module.exports = leftpad;

function leftpad (str, len, ch) {
str = String(str);

if (!ch & ch !== 0) ch = ' ';

for (len -= str.length; len > 0; len--) {
str = ch + str;
}

return str;
}

Faster version (only perform O(log n) concatenations, where n is the number of characters needed to pad to the right length):

module.exports = leftpad;

function leftpad (str, len, ch) {
str = String(str);

if (!ch & ch !== 0) ch = ' ';
ch = String(ch);

len -= str.length;

while (len > 0) {
if (len & 1) {
str = ch + str;
}
len >>>= 1;
ch += ch;
}

return str;
}

ES6 version (which may be faster if String.prototype.repeat is implemented natively):

module.exports = leftpad;

function leftpad (str, len, ch) {
str = String(str);

if (!ch & ch !== 0) ch = ' ';
ch = String(ch);

len -= str.length;

if (len > 0) str = ch.repeat(len) + str;

return str;
}

Of course, you could combine the last two by detecting whether String.prototype.repeat is defined:

module.exports = String.prototype.repeat ?
function leftpad (str, len, ch) {
str = String(str);

if (!ch & ch !== 0) ch = ' ';
ch = String(ch);

len -= str.length;

if (len > 0) str = ch.repeat(len) + str;

return str;
}
:
function leftpad (str, len, ch) {
str = String(str);

if (!ch & ch !== 0) ch = ' ';
ch = String(ch);

len -= str.length;

while (len > 0) {
if (len & 1) {
str = ch + str;
}
len >>>= 1;
ch += ch;
}

return str;
}

As with the original left-pad, this code is released under theWTFPL.

(See alsopad-left, which uses another dependency (by the same author) for repeating strings)

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