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Rounding numbers in Javascript


There are times when we need to round numbers to show on a web page. For example, we might want to round a number to two decimal places when showing currency values.

It is tempting just to truncate the values but that has its drawbacks. You may lose precision if you truncate the value without rounding it.

Enter toFixed(). This method takes a number and returns the rounded value to the number of places specified in the parameter.

The code below does the following:

  1. set up a constant holding the number we want to round
  2. use the toFixed() method to round the number to three decimal places
  3. log the result to the console
  4. log the type of object that the result is
const num1 = 7.74325156;
const result = num1.toFixed(3);
console.log(result); // => 7.743
console.log(typeof result); // => string

We get a surprise. Even though we used a number, running it through toFixed() returns a string.

To keep the value as a number after running it through toFixed(), we use the Number() function.

The Number function is part of the Number primitive wrapper object used to represent and manipulate numbers. In this case, we're converting the string in result2 to a number.

const num2 = 7.74325156;
const result2 = Number(num2.toFixed(3));
console.log(result); // => 7.743
console.log(typeof result2); // => number

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