numBeg.js

const numBegArr = ["-", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"];
const numBegSet = new Set(numBegArr);
const isNumericBeginning = char => numBegSet.has(char);
const isNumericBeginning2 = char => numBegArr.includes(char);

const isNumericBeginning3 = char =>
  char === "-" ||
  char === "0" ||
  char === "1" ||
  char === "2" ||
  char === "3" ||
  char === "4" ||
  char === "5" ||
  char === "6" ||
  char === "7" ||
  char === "8" ||
  char === "9";

const N = 1e6;
console.time("isNumericBeginning");
for (let i = 0; i < N; i++) {
  for (const char of "-0123456789") {
    isNumericBeginning(char);
  }
}
console.timeEnd("isNumericBeginning");

console.time("isNumericBeginning2");
for (let i = 0; i < N; i++) {
  for (const char of "-0123456789") {
    isNumericBeginning2(char);
  }
}
console.timeEnd("isNumericBeginning2");

console.time("isNumericBeginning3");
for (let i = 0; i < N; i++) {
  for (const char of "-0123456789") {
    isNumericBeginning3(char);
  }
}
console.timeEnd("isNumericBeginning3");