How to Format a Number to Always Show 2 Decimal Places in JavaScript

Formatting numbers to a consistent number of decimal places is often necessary in web development, especially when working with currencies, percentages, or other numeric data that benefits from standardized precision.

In JavaScript, there are a couple easy methods to format a number to always show two decimal places. The toFixed() and toLocaleString() methods provide simple ways to handle decimal rounding and string formatting without extensive coding.

In this comprehensive guide, we’ll explore these methods for enforcing two decimal places in JavaScript numbers. We’ll cover:

• Use cases showing when two decimal precision is helpful
• A breakdown of the toFixed() method and examples
• How to leverage toLocaleString() for number formatting
• Localization support with toLocaleString()
• Comparison of key differences in the methods
• Common formatting gotchas to avoid

Understanding these JavaScript number formatting approaches will equip you to handle rounding, precision, and readability for numbers in your web projects.

When to Format Numbers to Two Decimal Places

Showing two decimal places can improve consistency, precision, and readability for certain types of numeric data:

Currencies: When presenting monetary values, two decimals places properly represent cents/pence denominations in dollars, euros, pounds sterling, etc.

Percentages: Calculating percentages often involves decimal values. Showing two decimal places avoids confusion from rounding and enhances precision.

Weights and Measures: Systems like metric (grams, kilos, meters) and imperial (ounces, pounds, feet) often utilize two decimals even when fractional units are not involved.

General Readability: Numbers with long unrounded decimal values can be difficult to parse. Cutting down to two decimal places improves skimmability.

So in short, sticking to two decimal places aligns with conventions for various data types, prevents awkward long decimal numbers, and improves comprehension.

Next let’s look at how to implement two decimal rounding in JavaScript.

Using JavaScript’s toFixed() Method

The easiest way to format a number to always show two decimal places in JavaScript is with the toFixed() method.

toFixed() Overview

The toFixed() method is available on all JavaScript numbers (the Number prototype).

Calling .toFixed() on a number rounds the number to a specified number of decimals then returns the number as a string.

Here is the syntax:

num.toFixed(digits)

The digits parameter is the number of decimals to round to. So for two decimal places, we pass 2.

For example:

let num = 1.234567

num.toFixed(2) // Returns ‘1.23‘

Key things to note about toFixed():

• Rounds the number to the given number of decimals
• Returns a string, not a number type
• Useful for persisting a number to a certain precision without actually losing precision (since the original number value is untouched)

Next let’s look at some more robust examples of using toFixed() for enforcing two decimal place formatting.

toFixed() Examples

toFixed(2) makes an easy one-liner for displaying numbers with two decimals:

let price = 1.234 // = $1.23
price.toFixed(2) // "1.23" 

let temp = 22.45456 // = 22.45 degrees
temp.toFixed(2) // "22.45"

To use the returned string value, you can insert it into DOM elements:

let fees = 1.5748

document.getElementById("fee").innerText = fees.toFixed(2) 

// Inserts text "1.57" into DOM element

Or assign formatting to a variable:

let salesTaxPercent = 7.325

let taxString = salesTaxPercent.toFixed(2) // = "7.33"

document.getElementById("tax").innerText = taxString

One thing to note with toFixed() is that since it returns a string, if you want to use the value in further calculations, you’ll need to convert it back to a number using the Number() function:

let price = 10.895
let stringPrice = price.toFixed(2) // = "10.90" 

let total = Number(stringPrice) * 1.08 // Calculate total with tax

This parses the string back into a number that can be used mathematically.

toFixed() Gotchas

While toFixed() provides an easy method for decimal rounding, there are couple edge cases to keep in mind:

Limited Rounding Precision

toFixed() only supports rounding to a max of 20 digits after the decimal:

let num = 23.4792817391872971

num.toFixed(3) // Rounds fine - "23.479"  
num.toFixed(30) // FAILS - only supports 20 decimals 

So for full control beyond 20 decimals, different methods would be needed.

Rounding Errors

The internal rounding algorithm can sometimes introduce tiny errors from precision loss.

For example:

let x = 1.005 * 100 // = 100.49999999999999
let y = x.toFixed(2) // Rounds up to "100.50"

This tiny rounding up should be negligible in most cases but worth being aware of.

Overall though, used properly toFixed() provides an easy route to standard two decimal precision.

Leveraging JavaScript’s toLocaleString()

An alternative method for formatting numbers to a certain number of decimals is toLocaleString().

toLocaleString() Overview

The toLocaleString() method is also available on all JavaScript numbers. When called on a number it:

• Converts the number to a string, localized to the host environment’s locale and language
• Allows specifying options like minimum/maximum fraction digits to control decimal precision

Here is the basic syntax:

num.toLocaleString(locale, options) 

The method takes two optional parameters:

• locale – the locale to format the string towards
• options – object with formatting options like decimal precision

To simply format a number to two decimals, we can ignore locales and specify minimumFractionDigits and maximumFractionDigits like:

let num = 542.4315

num.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2})

// Returns "542.43" 

This handles truncating the number to two decimals for us.

Next let’s walk through some more examples using toLocaleString() for rounding decimals.

toLocaleString() Examples

Similarly to toFixed(), toLocaleString() can be used as quick one-liner:

let temp = 22.43222

temp.toLocaleString(undefined, {maximumFractionDigits: 2}) 

// Returns "22.43"

We can also use the returned string values by inserting them into the DOM:

let price = 19.99
let tax = 0.875

let total = price + tax

document.getElementById("total").innerText = total.toLocaleString(undefined, {
  minimumFractionDigits: 2,
  maximumFractionDigits: 2
})

// Inserts text "20.87" into DOM element

Or by assigning to string variables:

let padding = 2.33333

let padString = padding.toLocaleString(undefined, {maximumFractionDigits: 2}) 

// padString = "2.33"

For math operations, remember strings will again need conversion:

let sales = 100.75 

let salesString = sales.toLocaleString(undefined, {maximumFractionDigits: 2})

let fees = Number(salesString) * 0.0335 // Convert back to number

Localization Support

One extra advantage of toLocaleString() is it provides localization support for free.

We can specify a locale to automatically format the number string per that locale and language:

let price = 542.351

// German locale and decimals  
price.toLocaleString(‘de-DE‘, {maximumFractionDigits: 2})

// Returns "542,35" with commas for decimals

You can lookup valid locale tags, but common ones include:

• en-US – English (United States)
• en-GB – English (Britain)
• fr-FR – French
• zh-CN – Chinese
• etc.

This makes translating decimal formats much easier across languages.

toLocaleString() Gotchas

Similar to toFixed() be aware that toLocaleString():

• Returns string values that may need parsing for math ops
• Has limited rounding precision (~20 digits max)
• Could introduce tiny rounding errors

So these precision tradeoffs are worth keeping in mind for mission critical calculations.

toFixed() vs toLocaleString() Comparison

Now that we’ve covered toFixed() and toLocaleString() in detail, let’s recap some key differences between the methods:

Feature	toFixed()	toLocaleString()
Supported parameters	Digits only	Locale + options object
Localization	❌	✅
Output value	String	String
Rounding	Built-in algorithm	Truncate specified digits
Ease of use	Slightly easier	Little more verbose

So in summary:

• toFixed() is simpler with only digit rounding
• toLocaleString() handles locales but needs more options
• Both output strings so numbers may need re-parsing
• Either achieves standard two decimal formatting

Choosing between them depends if you need localization or fine-grained control over rounding rules.

Avoiding Common Number Format Pitfalls

While toFixed() and toLocaleString() abstract away much of the complexity around decimal formatting, there are still a couple easy mistakes to make. Be aware of:

Mixing Data Contexts

Don’t display currency and percentages using the same decimal precision:

// Confusing - are percentages or dollars!?
Sale: $6.50
Discount: 20.00% 

Stick to currency conventions like $6.50 and percentage conventions like 20.00%.

Hard-coded Precision

Avoid littering your code with hard-coded decimals:

// Hard to maintain
let taxRate = 0.0625
let discount = 0.10
// etc.

Instead, calculate then standardize precision:

let taxRate = 6.25 / 100  
let formattedTax = taxRate.toFixed(2) // "0.06"

let discount = 10 / 100
let formatDisc = discount.toFixed(2) // "0.10" 

This keeps the original values changeable.

Putting into Practice

The key takeaways are:

• Use toFixed(2) or toLocaleString() to easily format decimals
• Remember they return strings not numbers
• Be aware of precision limits and locale needs
• Avoid common decimal pitfalls

Applying these guidelines will properly handle rounding and formatting for currencies, percentages, metrics, and any numeric data requiring a consistent decimal display.

The techniques shown here provide a straightforward way to reinforce significant digits, improve readability, and standardize interfaces by formatting JavaScript numbers to always show two decimal places.

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I hope this guide gives you a comprehensive overview of formatting decimal numbers in JavaScript! Let me know if you have any other questions.