JavaScript Number type would have been called double or float64 in a compiled language. Therefore, numbers have some limits:

const maxInt = Number.MAX_SAFE_INTEGER; // 9007199254740991
const minInt = Number.MIN_SAFE_INTEGER; // -9007199254740991
minInt === -maxInt; // true
const maxDouble = Number.MAX_VALUE; // 1.7976931348623157e+308
const minDouble = Number.MIN_VALUE; // -1.7976931348623157e+308

See that strange long number in min and max values? That is the first magical way of representing a JavaScript number: using a base and an exponent (a.k.a beN):

const minusTwo = -2; // -2
const one75 = 175e-2; // 1.75

In practice, you specify a base number, then write e and specify where you want to move the dot. If the last number is positive - you'll be adding zeros, if negative - you'll be moving the dot to the left:

const zero = 0; // 0; move dot from 0.0 zero times
const zero5 = 5e-1; // 0.5; move dot from 5.0 one time left
const fifty = 5e1; // 50; move dot from 5.0 one time right
const alsoFifty = 5e1; // 50
const minusFifty = -5e1; //-50; move dot from -5.0 one time right
const seventeen5 = 1.75e1; // 17.5; move dot from 1.75 one time right

But, this representation may be a bit hard to read, so you can also use old plain decimals with the underscore as a separator:

const million = 1_000_000; //1000000
const fiveK = 5_000 // 5000
const justLoveUnderscores = 1_2_3.3_2_1 //123.321

const oops = 1__0 // Error: Only one underscore is allowed as numeric separator
const nope = 0_1 // Error: Numeric separator can not be used after leading 0

Dealing with binary numbers instead? No worries, you can write it like this:

const five = 0b101; // 5
const alsoFive = 0b101; // 5; `b` is case tolerant
const max8bit = 0b1111_1111; // 255; You can use separators in any number :-)
const lessFive = -0b0101; // -5

Of course, hexadecimal numbers are also a must-have in your arsenal:

const max8bit = 0xff; // 255
const hexNum = -0xabc; // -2748
const max32bit = 0xffff_ffff; // 4294967295; can use separators

And, just so you know, ECMA Script 2015 introduced octals:

const maxBit = 0o7; // 7
const eight = 0o10; // 8
const funnyZero = 0o0; // 0

If so happens and you can't squeeze your integer number in 64bits, you can convert it to BigInt by adding an n to it:

const maxDouble = 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff; // Infinity
const biggerThanMaxDouble =
  0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffn; // 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137215n

const bigTen = 10n; // 10n;
const bigLess50 = -50n; // -50n

And, there are several truly magic numbers in JavaScript:

(Infinity ===
  Number.POSITIVE_INFINITY - // true
    Infinity) ===
  Number.NEGATIVE_INFINITY; // true
const smallestFraction = Number.EPSILON; // 2.2204460492503130808472633361816e-16
smallestFraction === Math.pow(2, -52); // true

As everyone has that weird cousin, JavaScript numbers have a special number that is literally not a number.

NaN value is a special value, and every single operation with it will result in NaN, including comparison:

NaN === Number.NaN; // false !!
Number.isNaN(NaN); // true

NaN + 1; // NaN
NaN * 15; // NaN
NaN / NaN; // NaN
1 * NaN; // NaN

The most convinient way to make a NaN is through a failed type conversion:

parseInt("abc"); // NaN
parseInt({}); // NaN
parseFloat("1.1.1"); // NaN
"abc" * 1; // NaN
new Number("abc"); // Number {NaN}
Math.abs("abc");

However, there are built-in functions to help you deal with edge cases and NaN:

Number.isNaN(123); // false
Number.isNaN("abc"); // true
Number.isNaN(NaN); // true

Number.isFinite(123); // true
Number.isFinite(Infinity); // false
Number.isFinite(NaN); // false

Number.isInteger(123); // true
Number.isInteger(9007199254740992); // true !!
Number.isInteger(12.3); // false
Number.isInteger(Infinity); // false
Number.isInteger(NaN); // false

Number.isSafeInteger(123); // true
Number.isSafeInteger(9007199254740992); // false
Number.isSafeInteger(12.3); // false
Number.isSafeInteger(Infinity); // false
Number.isSafeInteger(NaN); // false

// BigInts are not considered Integers:
Number.isInteger(1n); // false
Number.isSafeInteger(1n); // false
// But it is a number:
Number.isNaN(1n); // false

Thirsty for more? Check out MDN Lexical grammar article.