Number .toFixed() / toPrecision()

// Converts to a string with the specified number of decimal places.
var str = number.toFixed(digits);

// Converts to a string with the specified number of significant digits.
var str = number.toPrecision(digits);

// Converts to a string in exponential notation.
var str = number.toExponential(digits);

var price = 1234.5678;

// toFixed() specifies the number of decimal places.
console.log(price.toFixed(2)); // Outputs "1234.57". Rounded at the third decimal place.
console.log(price.toFixed(0)); // Outputs "1235". Rounded to the nearest integer.
console.log(price.toFixed(6)); // Outputs "1234.567800". Padded with zeros to fill missing digits.

// toPrecision() specifies the total number of significant digits.
console.log(price.toPrecision(6)); // Outputs "1234.57". Six significant digits in total.
console.log(price.toPrecision(4)); // Outputs "1235". Four significant digits in total.
console.log(price.toPrecision(2)); // Outputs "1.2e+3". Switches to exponential notation when digits are insufficient.

// toExponential() converts to exponential notation.
console.log(price.toExponential(2)); // Outputs "1.23e+3".
console.log(price.toExponential()); // Outputs "1.2345678e+3".

// Practical example: displaying a price
var amount = 980;
console.log(amount.toFixed(2)); // Outputs "980.00". Useful for displaying prices with tax.

// Calculating sales tax
var tax = 980 * 0.1;
console.log(tax); // Outputs "98.00000000000001". Floating-point rounding error occurs.
console.log(tax.toFixed(0)); // Outputs "98". Round to an integer to eliminate the error.

number.toFixed() is the most commonly used number formatting method. It is useful whenever you need to align decimal places, such as when displaying prices or calculation results. Note that the return value is a string. If you need to use the result in further calculations, convert it back to a number using Number() or parseFloat().

JavaScript numbers are internally represented as IEEE 754 floating-point values, which can cause rounding errors — for example, 0.1 + 0.2 evaluates to 0.30000000000000004. number.toFixed() is commonly used to hide such errors when displaying values on screen.

number.toPrecision() controls the total number of significant digits, making it useful when the magnitude of a number is not known in advance and you want consistent precision. number.toExponential() is used for scientific calculations or when displaying very large or very small numbers.