JavaScript basic data type conversion

foreword

Data type conversion is a scenario we often encounter in front-end development. Generally speaking, there are three cases of type conversion in JavaScript:

• Convert to numbers (call Number(), parseInt(), parseFloat() methods)
• Convert to string (call .toString() or String() method)
• Convert to boolean (call Boolean() method)

It should be noted that: null, undefined does not have the .toString() method .

convert to number

Number(string) method

Number() can convert any value into a number. If the target object to be converted (usually a string) has a value that is not a number, it will return NaN .

Number('1')   // 1
Number(true)  // 1
Number('123s') // NaN
Number({})  //NaN

parseInt(string, radix)

Parses a string and returns a decimal integer in the specified radix, where radix is an integer between 2-36 representing the radix of the parsed string.

parseInt('2') //2
parseInt('2',10) // 2
parseInt('2',2)  // NaN
parseInt('a123')  // NaN  如果第一个字符不是数字或者符号就返回NaN
parseInt('123a')  // 123

parseFloat(string)

Parses an argument and returns a float.

parseFloat('123a')    //123
parseFloat('123a.01')    //123
parseFloat('123.01')    //123.01
parseFloat('123.01.1')    //123.01

implicit conversion

let str = '123'
-str    // -123
str+1 // '1231'
+str+1 // 124
let res = str - 1 //122

convert to string

toString()

Convert the target object to a string.

Note: null, undefined cannot be called.

Number(123).toString()    //'123'
[].toString()    //''
true.toString()    //'true'

String()

String() is more powerful, it can convert any incoming value into a string.

String(123)    //'123'
String(true)    //'true'
String([])    //''
String(null)    //'null'
String(undefined)    //'undefined'
String({})    //'[object Object]'

implicit conversion

When one of the two sides of + is a string and the other is another type, the other type will be converted to a string first, then the string will be concatenated, and the string will be returned:

let a = 1
a + ''    // '1'

convert to boolean

Boolean()

Boolean() method will convert the following values to false:

• 0
• ""
• null
• undefined
• NaN

All remaining values are converted to true.

Boolean('') //false
Boolean(0) //false
Boolean(1) //true
Boolean(null) //false
Boolean(undefined) //false
Boolean(NaN) //false
Boolean({}) //true
Boolean([]) //true

Conditional statements

In conditional statements, we generally do not actively perform boolean conversions.

let a
if(a) {
  //...   //这里a为undefined，会转为false，所以该条件语句内部不会执行
}

implicit conversion

let str = '111'
console.log(!!str) // true

Summarize

The above is a summary of the knowledge related to JavaScript basic data type conversion, of which the most important thing is hermit conversion.

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