Notes on Common Number Methods: Rounding, Tests with isNaN and isFinite, and parseInt()/parseFloat() Functions and Math Object

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Arithmetic Behavior in JavaScript

Arithmetic in JavaScript does not raise errors in cases of overflow, underflow, or division by zero. When the result of a numeric operation exceeds the largest representable number (overflow), the result is a special infinity value: Infinity.

Similarly, when the absolute value of a negative number exceeds the largest representable negative number, the result is -Infinity.

> Infinity      // too big number to represent
Infinity

> Number.POSITIVE_INFINITY  // same
Infinity

> 1 / 0
Infinity

> Number.MAX_VALUE      
1.7976931348623157e+308

> Number.MAX_VALUE * 2      // Overflow
Infinity

> -Infinity
-Infinity

> Number.NEGATIVE_INFINITY     // same
-Infinity

> -1 / 0
-Infinity

> -Number.MAX_VALUE
-1.7976931348623157e+308

> -Number.MAX_VALUE * 2
-Infinity

> 0 / 0
NaN

> Infinity / -Infinity
NaN

> Infinity / Infinity
NaN

Zero divided by zero does not have a well-defined value, and the result of this operation is the special "Not-a-Number" value, NaN. NaN also arises when attempting to divide infinity by infinity, take the square root of a negative number, or use arithmetic operators with non-numeric operands that cannot be converted to numbers.

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Number Properties in ES6

// The following Number properties are defined in ES6:

Number.parseInt()       
// Same as the global parseInt() function

Number.parseFloat()     
// Same as the global parseFloat() function

Number.isNaN(x)         
// Is x the NaN value?

Number.isFinite(x)      
// Is x a number and finite?

Number.isInteger(x)     
// Is x an integer?

Number.isSafeInteger(x) 
// Is x an integer -(2**53) < x < 2**53?

Number.MIN_SAFE_INTEGER  
// => -(2**53 - 1)

Number.MAX_SAFE_INTEGER  
// => 2**53 - 1

Number.EPSILON           
// => 2**-52: smallest difference between numbers

Testing for NaN and Finite Numbers

Arithmetic Behavior

Division by Zero: When dividing by zero, JavaScript returns Infinity or -Infinity depending on the sign.

alert(1 / 0);  // Infinity
alert(-1 / 0); // -Infinity

NaN (Not a Number): If an operation results in a value that cannot be calculated, it returns NaN (Not a Number). This can happen in operations like division by zero, or invalid operations involving non-numeric values.

alert("not a number" / 2);  // NaN
alert(NaN + 1);  // NaN

Special Cases Involving NaN:

• Any expression involving NaN will propagate NaN as the result:

0 / 0;           // NaN
Infinity - Infinity;  // NaN

• NaN Propagation: If NaN is part of an expression, the result will also be NaN, unless the operation is NaN raised to the power of 0, which is 1.

NaN ** 0;   // 1

JavaScript provides special functions to handle cases where values may not be valid numbers or may be infinite. The Not-a-Number value has one unusual feature in JavaScript: it does not compare equal to any other value, including itself.

This means that you can’t write x === NaN to determine whether the value of a variable x is NaN. Instead, you must write x != x or Number.isNaN(x). These expressions will be true if, and only if, x has the same value as the global constant NaN.

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'isNaN()'

The isNaN() function returns true if its argument is NaN, or if that argument is a non-numeric value that cannot be converted to a number. It first converts the argument to a number and then checks if it is NaN.

alert(isNaN(NaN));    
// true

alert(isNaN("str"));  
// true (because "str" is not a number)

alert(NaN === NaN);   
// false (NaN is not equal to itself)

'isFinite()'

The isFinite() function tests whether a value is a finite number (i.e., not NaN, Infinity, or -Infinity).

alert(isFinite("15"));      
// true (string "15" is converted to number)

alert(isFinite("str"));     
// false (because "str" is not a number)

alert(isFinite(Infinity));  // false
alert(isFinite(-Infinity)); // false

isFinite() is often used to validate user input to ensure it's a regular number:

let num = +prompt("Enter a number", '');
alert(isFinite(num));  
// true unless the value is NaN or Infinity

Converting Strings to Numbers: 'parseInt()' and 'parseFloat()'

JavaScript provides two built-in functions to convert strings into numeric values: parseInt() and parseFloat().

• parseInt(): Converts a string to an integer (whole number).
• parseFloat(): Converts a string to a floating-point number (decimal number).

'parseInt()'

parseInt() parses a string and returns an integer. It reads the string until it encounters a non-digit character, then stops parsing and returns the integer.

alert(parseInt('100px'));  // 100
alert(parseInt('12.5em')); // 12 (stops parsing at the first non-numeric character)
alert(parseInt('12.3'));   // 12 (only integer part is parsed)
alert(parseInt('a123'));   // NaN (cannot parse anything)

'parseFloat()'

parseFloat() parses a string and returns a floating-point number. It reads until it encounters a character that is not part of a floating-point number (e.g., a letter).

alert(parseFloat('12.5em')); // 12.5
alert(parseFloat('12.3.4')); // 12.3 (stops parsing at the second dot)
alert(parseFloat('100px'));  // 100
alert(parseFloat('a123'));   // NaN

Numeric Conversion Using + or 'Number()'

For strict conversion of a string to a number, you can use the unary plus (+) or Number() function. These will fail if the string cannot be strictly converted to a valid number.

alert(+"100px"); // NaN (invalid string)
alert(Number("100px")); // NaN

alert(+"123");  // 123 (valid number)
alert(Number("123")); // 123

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