JavaScript: Basic data type 1: Null, Undefined, Boolean, Number

JavaScript: Basic data type 1: Null, Undefined, Boolean, Number

• There are currently six basic data types in JavaScript :Boolean, String, Number, NULL, undefind, and Symbol

Null and undefined are special data types in JS that have only one value and are the simplest data types in JS.

• Undefined means that no value is defined (initialized)
  • Declaring that a variable is not initialized using var or let is equivalent to assigning undpay to the variable
• Null is an Object with a null pointer, which is why Typeof NULL returns “Object”
  • When defining the variable to be saved as an object value, it is recommended to use unLL for initialization
  • Undefined is derived from null, which is defined to indicate equality

      console.log(null == undefined) // true
      console.log(null === undefined) // false
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Boolean has two values true and false

• You can use Boolean() to convert any value to a Boolean value
• False: +0, -0, null, undefined,NaN, empty string (“”)
• True: All but the above initial values are true, including empty objects, empty arrays, the string “false”, non-zero values (including positive and negative infinity)

Number, Bigint,

The number types in JS use IEEE754’s 64-bit floating-point type: the largest safe integer, 2 to the 53th power -1

• Use:

    new Number(value);
    var a = new Number('123'); // a === 123 is false
    var b = Number('123'); // b === 123 is true
    a instanceof Number; // is true
    b instanceof Number; // is false
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• Floating point value

  • The decimal point is not necessarily preceded by an integer, but it is recommended

  Let floatNum1 = 1.1 let floatNum2 = 0.1 Let floatNum3 =.1 // Valid single not recommended Let floatNum4 = 1 Treat as integerCopy the code

  • Floating-point numbers are as accurate as 17 as a decimal but far less accurate in arithmetic than integers

  The rounding error is due to the use of IEEE754 values and is not unique to JS

  // 0.1+0.2 does not equal 0.3 0.1+0.2=0.300 000 000 000 000 04Copy the code

• Scope of worth

  • MIN_VALUE In most browsers, the value is 5E-324-infinity
  • Maximum value: number. MAX_VALUE Converts to Infinity beyond this range
  • Check whether isFinite() is used between maximum and minimum values

• NaN

  • Any operation that involves NaN always returns NaN
  • NaN is not equal to any value including NaN. You can use the isNaN function to determine if a value isNaN

  console.log(0/0)  // NaN
  console.log(-0/+0) // NaN
  console.log(5/0) // Infinity
  console.log(5/-0) // - Infinity
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• Conversion of values

  • There are three methods for converting numeric types: Number(), parseInt(), parseFloat()
  • Number() executes the transformation rule
    • Boolean true -> 1; false -> 0
    • Return value directly
    • Null, returns 0
    • undefined -> NaN
    • String:
      • Contains numeric values (including plus and minus signs) to decimal values ignoring the preceding 0
      • Contains valid floating-point numbers converted to corresponding integers, and the decimal point is not a valid value
      • Contains hexadecimal formats such as “0xf” converted to a hexadecimal integer
      • An empty string returns 0
      • In other cases NaN is returned
    • Object that calls valueOf() and then converts as described above, or toString() if NaN is returned
  • ParseInt () is more focused on whether the string contains a numeric pattern
    • Ignore the preceding whitespace, starting with the first non-null character: not a numeric value (which can contain a plus or minus sign) returns NaN
    • An empty string returns NaN
    • If the first character is a numeric value (which can contain a plus or minus sign), the sequence is matched, returning until a non-numeric character is encountered
    • You can pass in a second argument to specify base (base)

    ParseInt (" 1234blue") // returns 1234 parseInt("22.5") // Returns 22 decimal point is not a valid number parseInt("") // NaN parseInt("0xA") // 10 is interpreted in hexadecimal ParseInt ("AF", 16) // Hexadecimal 175 parseInt("AF") // NaN parseInt("10", 2) // parseInt in binaryCopy the code

  • ParseFloat () is similar to parseInt() but only parses decimal

  ParseFloat ("0.43Bul") // returns 0.43 parseFloat("22.34.5") // returns 22.34Copy the code

reference

• MDN: developer.mozilla.org/zh-CN/docs/…
• Books: The Little Red Book 3rd & 4th editions – javascript Advanced Programming