Question 219025 in Html Question posted in
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Html – What is the standard way to check a variable is initialised as a number in javascript?

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];
Tags:

2

Answers


  1. 0

    Try this:

    var i;

if(!!i) {
   console.log("undefined or null or (falsy) ");
}
else {
   console.log("The value of i is ", i);
}
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  2. 0

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

    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 !isNaN(Number(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));
}
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