What is the standard way to check a variable is initialised as a number in javascript?

Question

I am trying to test a variable to ensure [a] it is initialised and [b] and is a number before supplying the number as an argument. I have tried to do this using two functions noNil(i) and noNan(i). It makes sense to find if a variable is initialised before testing if it is a number. But it makes little difference which function is called first. The first alert reports undefined while the second alert report nothing. What is the standard way to check a variable is initialised as a number in pure javascript ?

e.g.

    var i;

    noNil();
    alert(i);
    noNaN(i);
    alert(i);

    function noNil(i) {
      if (i === undefined){
      let i = -1;
        return i;
      }
    }

    function noNan(i) {         
      if (isNaN(i)) {
      let i = 30;   
        return i;
      }
    }

Clarification

A bit of context might help. The variable is defined in 5 ways; 0 to 4 select colours from an array. If it is undefined none of the colours is selected.

    let family    = ["yellow", "red", "lime", "cyan","magenta"];
    ...
    ctx.fillStyle = family[i];

Answer 1

Use typeof i === 'number' to determine whether the value is of the JS number type.

const checkNumber = i => { if( typeof i !== 'number') throw new Error('Supplied argument is not a number!'); return i;};

function sum(a, b){
  return checkNumber(a) + checkNumber(b);
}

console.log(sum(2, 2));

var i;

console.log(sum(i, 2));

About your use case with selecting colors, you could avoid bothering with numbers:

let family    = ["yellow", "red", "lime", "cyan","magenta"];
family[i] && (ctx.fillStyle = family[i]);

But the main question could be why you are ending up with i not being a number of a proper index of your color array...

If you want to check strings the topic suddenly becomes much more complex (I don't even cover all cases here like scientific notation):

var i;

function isNumber(i) {
  return typeof i === 'number';
}

function isNumerish(i) {
  return !isNaN(parseFloat(i));
}

function isStringNumber(i) {
  return /^-?\d*\.?\d+$/.test(i);
}

const checks = [[i, isNumber], [-.5, isNumber], ['-.5a', isNumerish], ['-.5a', isStringNumber], ['-.5', isStringNumber]];

for(const [num, fn] of checks){
  console.log(num, fn.name, fn(num));
}