fracs.js

let fractionCodes = {
    189: "1/2",
    8531: "1/3", 
    8532: "2/3",
    188: "1/4",
    190: "3/4",
    8533: "1/5"
}; //fill in the rest from unicodefractions.com

function removeFractions(s) {
    for (let [code, str] of Object.entries(fractionCodes)) {
        let c = String.fromCharCode(code);
        s = s.replace(c, str);
    }
    return s;
}

let s1 = " ¾ a pinch of ⅓ a lemon";
let s2 = "This doesn't start with a number";
let s3 = "1 1/2 cups stuff";

s1 = removeFractions(s1);
s2 = removeFractions(s2);
s3 = removeFractions(s3);
console.log(s1);
console.log(s2);
console.log(s3);

const FRAC_RE = /^\s*(\d+)?\s+(\d+)\s*\/\s*(\d+)\s+/;

console.log(FRAC_RE.exec(s1));
console.log(FRAC_RE.exec(s2));
console.log(FRAC_RE.exec(s3));


function convertFractions(s) {
    let result = FRAC_RE.exec(s);
    if (result) {
        let match = result[0];
        let whole = result[1];
        let numerator = result[2];
        let denominator = result[3];

        let decimal = Number(whole) || 0;
        decimal += numerator / denominator;

        return s.replace(match, `${decimal} `);
    } else {
        return s;
    }
}

s1 = convertFractions(s1);
s2 = convertFractions(s2);
s3 = convertFractions(s3);

console.log(s1);
console.log(s2);
console.log(s3);

:)