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Round

One of the most used operations, when working with numbers, is rounding. 
 
/**
 * Several built-in functions for rounding
 */

let n = 1.23456;
let a = Math.round(n * 100) / 100;  // 123 -> 1.23
let b = n.toFixed(2);               // "1.23"

console.log(a);             // 1.23
console.log(1.23 === b);    // false

Precision

Internally, a number is represented in a 64-bit format. 
 
/**
 * Losing precision is quite common
 * and this is true for many other languages
 * 
 * There are exactly 64 bits to store a number:
 *      52 to store the digits
 *      11 to store the position of decimal point
 *      1 bit for the sign
 */

let sum = 0.1 + 0.2;

console.log(sum); // 0.30000000000000004
console.log(sum.toFixed(2) === "0.30"); // true
console.log(+sum.toFixed(2) === 0.3); // true (unary + converts to number)

Conversion

Parse functions read a number from a string, until they can't. 
 
/**
 * In real life, values like "100px" or "19$"
 * 
 * parseInt() returns integer
 * parserFloat() returns floating-point number 
 */

console.log(parseInt("100px") === 100);     // true
console.log(parseInt("19$") === 19);        // true
console.log(parseFloat("12.3") === 12.3);   // true
console.log(parseFloat("12.3.4") === 12.3); // true
    // 12.3 second point stop reading



  Last update: 357 days ago