I wish I could log to the console the result of a multiplication of a number by a variable but I get NaN as a result while using Javascript.

My attempt:

var div = Number(console.log(10 / 50)); //returns 0.2
typeof div //number
console.log(div * 100); //returns NaN

I also tried

var div = Number(console.log(10 / 50)); //returns 0.2
typeof div //number
console.log(Number(div) * 100); //returns NaN

Why is it happening and how should I avoid it?

I wish I could log to the console the result of a multiplication of a number by a variable but I get NaN as a result while using Javascript.

My attempt:

var div = Number(console.log(10 / 50)); //returns 0.2
typeof div //number
console.log(div * 100); //returns NaN

I also tried

var div = Number(console.log(10 / 50)); //returns 0.2
typeof div //number
console.log(Number(div) * 100); //returns NaN

Why is it happening and how should I avoid it?

• javascript

• You could define your own function tap = x => (console.log(x), x) and use it instead of console.log var div = Number(tap(10 / 50)); – Yury Tarabanko Commented Aug 14, 2017 at 14:11
• Thanks, but what I was looking for required me to log a step by step result. That solution would give me only the final result. – pedrez Commented Aug 14, 2017 at 14:36
• It will give whatever result you use this function on. – Yury Tarabanko Commented Aug 14, 2017 at 14:40

4 Answers 4

The console.log() function returns undefined, so Number(console.log(anything)) will always be NaN. Your code assumes that console.log() returns the value that it wrote to the browser developer console, but that is not the case.

Also, probably counter intuitively, NaN is a number, so typeof NaN is "number". Your code tested the return value from Number(console.log()) and it was in fact a number, but that number is NaN.

console.log only display the result and returns undefined.

Try directly:

 var div = Number(10 / 50);

var div = Number(10 / 50); //returns 0.2
typeof div //number
console.log(div * 100); 

console.log() does not return a number, and so you get a NaN error.

You can instead do:

var div = Number(10 / 50);
console.log(div * 100);

Hope this helps!

A small workaround:

 function log(...values){
  console.log(...values);
  return values.length === 1 ? values[0] : values;
}

So you can do:

var a = log(5/10)*2;
log(a);
log(log(1,2)[0]+3)

Alternatively, do it the other way round:

 var a;

console.log(
  a = 10/50,
  10*a,
  11*a
);