JavaScript has a number of built-in methods for working with numbers, including the ability to round them. In this article, we’ll cover how to round down numbers in JavaScript, also known as truncating decimals.
Truncating Decimals with Math.trunc()
One way to round down a number in JavaScript is to use the Math.trunc() method. This method returns the integer part of a number by removing any fractional digits.
For example, to round down the number 4.7 to 4, you can use the following code:
let num = 4.7;
let roundedDown = Math.trunc(num); // 4
Code language: JavaScript (javascript)The Math.trunc() method works with both positive and negative numbers, and can also be used on variables or expressions.
let positiveNum = 5.9;
let negativeNum = -5.9;
let expression = -(5.9 + 4.2);
console.log(Math.trunc(positiveNum)); // 5
console.log(Math.trunc(negativeNum)); // -5
console.log(Math.trunc(expression)); // -10
Code language: JavaScript (javascript)Using parseInt() to Round Down
Another way to round down numbers in JavaScript is to use the parseInt() function. This function parses a string argument and returns an integer. When used with a decimal number, parseInt() will round down to the nearest integer.
For example:
let positiveNum = 5.9;
let negativeNum = -5.9;
let expression = -(5.9 + 4.2);
console.log(parseInt(positiveNum)); // 5
console.log(parseInt(negativeNum)); // -5
console.log(parseInt(expression)); // -10
Code language: JavaScript (javascript)Note: parseInt() works on strings but if you pass a number then it will first be converted to a string using String() and then will be converted an integer.
Rounding Down with Bitwise Operators
Another option for rounding down numbers in JavaScript is to use bitwise operators. Specifically, the | (bitwise OR) operator can be used to truncate decimals.
To round down a number using bitwise operators, you can use the following code:
let num = 4.7;
let roundedDown = num | 0; // 4
Code language: JavaScript (javascript)This works by converting the number to a 32-bit integer and then back to a JavaScript number. Note that this method works with both positive and negative numbers, as the bitwise OR operator will truncate the decimal regardless of the sign of the number.
Choosing the Right Method for Your Needs
When deciding which method to use for rounding down numbers in JavaScript, it’s important to consider your specific needs and which method is most appropriate for your situation.
Here’s a quick comparison of the three methods discussed in this article:
| Method | Pros | Cons |
|---|---|---|
Math.trunc() | Simple and easy to use | Not supported in older browsers (IE and Edge versions prior to IE11 and Edge 79) |
parseInt() | Works with both positive and negative numbers | Requires conversion to a string before use |
| Bitwise operators | Fast and efficient | Terse, hard to understand |
In most cases, Math.trunc() is the easiest and most straightforward method for rounding down numbers in JavaScript. It’s supported in modern browsers, and is simple to use. However, if you need to support older browsers or if you need a method that works with both positive and negative numbers, parseInt() or bitwise operators may be a better choice.
Conclusion
In this article, we’ve covered three different ways to round down numbers in JavaScript: using Math.trunc(), parseInt(), and bitwise operators. Each method has its own pros and cons, and the right choice will depend on your specific needs and the compatibility requirements of your code.
Regardless of which method you choose, it’s important to keep in mind that rounding down numbers is just one of the many ways you can work with numbers in JavaScript. With a little bit of practice and a solid understanding of the available methods, you’ll be able to manipulate numbers with ease in your code.
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