JSFiddle - React, Tailwind, and code Playground

Babel + JSX

const left_pad = (s,l,c=' ')=>c.repeat(l-s.length)+s


const tests = [
left_pad("foo", 5), // "  foo"
left_pad("foobar", 6), // "foobar"
left_pad("1", 2, "0"), // "01"
left_pad("1", 2, "-"), // "-1"
];

for (let testCase of tests) {
console.log(testCase);
}